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<h1 class="chapter" id="c:intf-c">Chapter&#XA0;20&#XA0;&#XA0;Interfacing C with OCaml</h1>
<ul>
<li><a href="intfc.html#sec413">20.1&#XA0;&#XA0;Overview and compilation information</a>
</li><li><a href="intfc.html#sec420">20.2&#XA0;&#XA0;The <span class="c003">value</span> type</a>
</li><li><a href="intfc.html#sec424">20.3&#XA0;&#XA0;Representation of OCaml data types</a>
</li><li><a href="intfc.html#sec431">20.4&#XA0;&#XA0;Operations on values</a>
</li><li><a href="intfc.html#sec439">20.5&#XA0;&#XA0;Living in harmony with the garbage collector</a>
</li><li><a href="intfc.html#sec447">20.6&#XA0;&#XA0;A complete example</a>
</li><li><a href="intfc.html#sec448">20.7&#XA0;&#XA0;Advanced topic: callbacks from C to OCaml</a>
</li><li><a href="intfc.html#sec458">20.8&#XA0;&#XA0;Advanced example with callbacks</a>
</li><li><a href="intfc.html#sec459">20.9&#XA0;&#XA0;Advanced topic: custom blocks</a>
</li><li><a href="intfc.html#sec466">20.10&#XA0;&#XA0;Advanced topic: cheaper C call</a>
</li><li><a href="intfc.html#sec470">20.11&#XA0;&#XA0;Advanced topic: multithreading</a>
</li><li><a href="intfc.html#sec473">20.12&#XA0;&#XA0;Advanced topic: interfacing with Windows Unicode APIs</a>
</li><li><a href="intfc.html#sec475">20.13&#XA0;&#XA0;Building mixed C/OCaml libraries: <span class="c003">ocamlmklib</span></a>
</li></ul>
<p>This chapter describes how user-defined primitives, written in C, can
be linked with OCaml code and called from OCaml functions, and how
these C functions can call back to OCaml code.</p>
<h2 class="section" id="sec413">20.1&#XA0;&#XA0;Overview and compilation information</h2>
<h3 class="subsection" id="sec414">20.1.1&#XA0;&#XA0;Declaring primitives</h3>
<table class="display dcenter"><tr class="c019"><td class="dcell"><table class="c001 cellpading0"><tr><td class="c018">
<a class="syntax" href="modules.html#definition"><span class="c010">definition</span></a></td><td class="c015">::=</td><td class="c017">&#XA0;...
&#XA0;</td></tr>
<tr><td class="c018">&nbsp;</td><td class="c015">&#X2223;</td><td class="c017">&#XA0;<span class="c004">external</span>&#XA0;<a class="syntax" href="names.html#value-name"><span class="c010">value-name</span></a>&#XA0;<span class="c004">:</span>&#XA0;&#XA0;<a class="syntax" href="types.html#typexpr"><span class="c010">typexpr</span></a>&#XA0;<span class="c004">=</span>&#XA0;&#XA0;<a class="syntax" href="#external-declaration"><span class="c010">external-declaration</span></a>
&#XA0;</td></tr>
<tr><td class="c018">&nbsp;</td></tr>
<tr><td class="c018">
<a class="syntax" id="external-declaration"><span class="c010">external-declaration</span></a></td><td class="c015">::=</td><td class="c017">&#XA0;<a class="syntax" href="lex.html#string-literal"><span class="c010">string-literal</span></a>&#XA0;&#XA0;[&#XA0;<a class="syntax" href="lex.html#string-literal"><span class="c010">string-literal</span></a>&#XA0;&#XA0;[&#XA0;<a class="syntax" href="lex.html#string-literal"><span class="c010">string-literal</span></a>&#XA0;]&#XA0;]
</td></tr>
</table></td></tr>
</table><p>User primitives are declared in an implementation file or
<span class="c004">struct</span>&#X2026;<span class="c004">end</span> module expression using the <span class="c004">external</span> keyword:
</p><pre>
        external <span class="c009">name</span> : <span class="c009">type</span> = <span class="c009">C-function-name</span>
</pre><p>
This defines the value name <span class="c009">name</span> as a function with type
<span class="c009">type</span> that executes by calling the given C function.
For instance, here is how the <span class="c003">input</span> primitive is declared in the
standard library module <span class="c003">Pervasives</span>:
</p><pre>        external input : in_channel -&gt; bytes -&gt; int -&gt; int -&gt; int
                       = "input"
</pre><p>Primitives with several arguments are always curried. The C function
does not necessarily have the same name as the ML function.</p><p>External functions thus defined can be specified in interface files or
<span class="c004">sig</span>&#X2026;<span class="c004">end</span> signatures either as regular values
</p><pre>
        val <span class="c009">name</span> : <span class="c009">type</span>
</pre><p>
thus hiding their implementation as C functions, or explicitly as
&#X201C;manifest&#X201D; external functions
</p><pre>
        external <span class="c009">name</span> : <span class="c009">type</span> = <span class="c009">C-function-name</span>
</pre><p>
The latter is slightly more efficient, as it allows clients of the
module to call directly the C function instead of going through the
corresponding OCaml function. On the other hand, it should not be used
in library modules if they have side-effects at toplevel, as this
direct call interferes with the linker&#X2019;s algorithm for removing unused
modules from libraries at link-time.</p><p>The arity (number of arguments) of a primitive is automatically
determined from its OCaml type in the <span class="c003">external</span> declaration, by
counting the number of function arrows in the type. For instance,
<span class="c003">input</span> above has arity 4, and the <span class="c003">input</span> C function is called with
four arguments. Similarly,
</p><pre>    external input2 : in_channel * bytes * int * int -&gt; int = "input2"
</pre><p>has arity 1, and the <span class="c003">input2</span> C function receives one argument (which
is a quadruple of OCaml values).</p><p>Type abbreviations are not expanded when determining the arity of a
primitive. For instance,
</p><pre>        type int_endo = int -&gt; int
        external f : int_endo -&gt; int_endo = "f"
        external g : (int -&gt; int) -&gt; (int -&gt; int) = "f"
</pre><p><span class="c003">f</span> has arity 1, but <span class="c003">g</span> has arity 2. This allows a primitive to
return a functional value (as in the <span class="c003">f</span> example above): just remember
to name the functional return type in a type abbreviation.</p><p>The language accepts external declarations with one or two
flag strings in addition to the C function&#X2019;s name. These flags are
reserved for the implementation of the standard library.</p>
<h3 class="subsection" id="sec415">20.1.2&#XA0;&#XA0;Implementing primitives</h3>
<p>User primitives with arity <span class="c009">n</span> &#X2264; 5 are implemented by C functions
that take <span class="c009">n</span> arguments of type <span class="c003">value</span>, and return a result of type
<span class="c003">value</span>. The type <span class="c003">value</span> is the type of the representations for OCaml
values. It encodes objects of several base types (integers,
floating-point numbers, strings,&#XA0;&#X2026;) as well as OCaml data
structures. The type <span class="c003">value</span> and the associated conversion
functions and macros are described in detail below. For instance,
here is the declaration for the C function implementing the <span class="c003">input</span>
primitive:
</p><pre>CAMLprim value input(value channel, value buffer, value offset, value length)
{
  ...
}
</pre><p>When the primitive function is applied in an OCaml program, the C
function is called with the values of the expressions to which the
primitive is applied as arguments. The value returned by the function is
passed back to the OCaml program as the result of the function
application.</p><p>User primitives with arity greater than 5 should be implemented by two
C functions. The first function, to be used in conjunction with the
bytecode compiler <span class="c003">ocamlc</span>, receives two arguments: a pointer to an
array of OCaml values (the values for the arguments), and an
integer which is the number of arguments provided. The other function,
to be used in conjunction with the native-code compiler <span class="c003">ocamlopt</span>,
takes its arguments directly. For instance, here are the two C
functions for the 7-argument primitive <span class="c003">Nat.add_nat</span>:
</p><pre>CAMLprim value add_nat_native(value nat1, value ofs1, value len1,
                              value nat2, value ofs2, value len2,
                              value carry_in)
{
  ...
}
CAMLprim value add_nat_bytecode(value * argv, int argn)
{
  return add_nat_native(argv[0], argv[1], argv[2], argv[3],
                        argv[4], argv[5], argv[6]);
}
</pre><p>The names of the two C functions must be given in the primitive
declaration, as follows:
</p><pre>
        external <span class="c009">name</span> : <span class="c009">type</span> =
                 <span class="c009">bytecode-C-function-name native-code-C-function-name</span>
</pre><p>
For instance, in the case of <span class="c003">add_nat</span>, the declaration is:
</p><pre>        external add_nat: nat -&gt; int -&gt; int -&gt; nat -&gt; int -&gt; int -&gt; int -&gt; int
                        = "add_nat_bytecode" "add_nat_native"
</pre><p>
Implementing a user primitive is actually two separate tasks: on the
one hand, decoding the arguments to extract C values from the given
OCaml values, and encoding the return value as an OCaml
value; on the other hand, actually computing the result from the arguments.
Except for very simple primitives, it is often preferable to have two
distinct C functions to implement these two tasks. The first function
actually implements the primitive, taking native C values as
arguments and returning a native C value. The second function,
often called the &#X201C;stub code&#X201D;, is a simple wrapper around the first
function that converts its arguments from OCaml values to C values,
call the first function, and convert the returned C value to OCaml
value. For instance, here is the stub code for the <span class="c003">input</span>
primitive:
</p><pre>CAMLprim value input(value channel, value buffer, value offset, value length)
{
  return Val_long(getblock((struct channel *) channel,
                           &amp;Byte(buffer, Long_val(offset)),
                           Long_val(length)));
}
</pre><p>(Here, <span class="c003">Val_long</span>, <span class="c003">Long_val</span> and so on are conversion macros for the
type <span class="c003">value</span>, that will be described later. The <span class="c003">CAMLprim</span> macro
expands to the required compiler directives to ensure that the
function is exported and accessible from OCaml.)
The hard work is performed by the function <span class="c003">getblock</span>, which is
declared as:
</p><pre>long getblock(struct channel * channel, char * p, long n)
{
  ...
}
</pre><p>
To write C code that operates on OCaml values, the following
include files are provided:
</p><div class="center"><table class="c000 cellpadding1" border=1><tr><td class="c014"><span class="c013">Include file</span></td><td class="c014"><span class="c013">Provides</span> </td></tr>
<tr><td class="c022">
<span class="c003">caml/mlvalues.h</span></td><td class="c021">definition of the <span class="c003">value</span> type, and conversion
macros </td></tr>
<tr><td class="c022"><span class="c003">caml/alloc.h</span></td><td class="c021">allocation functions (to create structured OCaml
objects) </td></tr>
<tr><td class="c022"><span class="c003">caml/memory.h</span></td><td class="c021">miscellaneous memory-related functions
and macros (for GC interface, in-place modification of structures, etc). </td></tr>
<tr><td class="c022"><span class="c003">caml/fail.h</span></td><td class="c021">functions for raising exceptions
(see section&#XA0;<a href="#s%3Ac-exceptions">20.4.5</a>) </td></tr>
<tr><td class="c022"><span class="c003">caml/callback.h</span></td><td class="c021">callback from C to OCaml (see
section&#XA0;<a href="#s%3Acallback">20.7</a>). </td></tr>
<tr><td class="c022"><span class="c003">caml/custom.h</span></td><td class="c021">operations on custom blocks (see
section&#XA0;<a href="#s%3Acustom">20.9</a>). </td></tr>
<tr><td class="c022"><span class="c003">caml/intext.h</span></td><td class="c021">operations for writing user-defined
serialization and deserialization functions for custom blocks
(see section&#XA0;<a href="#s%3Acustom">20.9</a>). </td></tr>
<tr><td class="c022"><span class="c003">caml/threads.h</span></td><td class="c021">operations for interfacing in the presence
of multiple threads (see section&#XA0;<a href="#s%3AC-multithreading">20.11</a>). </td></tr>
</table></div><p>
These files reside in the <span class="c003">caml/</span> subdirectory of the OCaml
standard library directory, which is returned by the command
<span class="c003">ocamlc -where</span> (usually <span class="c003">/usr/local/lib/ocaml</span> or <span class="c003">/usr/lib/ocaml</span>).</p><p>By default, header files in the <span class="c003">caml/</span> subdirectory give only access
to the public interface of the OCaml runtime. It is possible to define
the macro <span class="c003">CAML_INTERNALS</span> to get access to a lower-level interface,
but this lower-level interface is more likely to change and break
programs that use it.</p><p><span class="c013">Note:</span> It is recommended to define the macro <span class="c003">CAML_NAME_SPACE</span>
before including these header files. If you do not define it, the
header files will also define short names (without the <span class="c003">caml_</span> prefix)
for most functions, which usually produce clashes with names defined
by other C libraries that you might use. Including the header files
without <span class="c003">CAML_NAME_SPACE</span> is only supported for backward
compatibility.</p>
<h3 class="subsection" id="sec416">20.1.3&#XA0;&#XA0;Statically linking C code with OCaml code</h3>
<p>
<a id="staticlink-c-code"></a></p><p>The OCaml runtime system comprises three main parts: the bytecode
interpreter, the memory manager, and a set of C functions that
implement the primitive operations. Some bytecode instructions are
provided to call these C functions, designated by their offset in a
table of functions (the table of primitives).</p><p>In the default mode, the OCaml linker produces bytecode for the
standard runtime system, with a standard set of primitives. References
to primitives that are not in this standard set result in the
&#X201C;unavailable C primitive&#X201D; error. (Unless dynamic loading of C
libraries is supported &#X2013; see section&#XA0;<a href="#dynlink-c-code">20.1.4</a> below.)</p><p>In the &#X201C;custom runtime&#X201D; mode, the OCaml linker scans the
object files and determines the set of required primitives. Then, it
builds a suitable runtime system, by calling the native code linker with:
</p><ul class="itemize"><li class="li-itemize">
the table of the required primitives;
</li><li class="li-itemize">a library that provides the bytecode interpreter, the
memory manager, and the standard primitives;
</li><li class="li-itemize">libraries and object code files (<span class="c003">.o</span> files) mentioned on the
command line for the OCaml linker, that provide implementations
for the user&#X2019;s primitives.
</li></ul><p>
This builds a runtime system with the required primitives. The OCaml
linker generates bytecode for this custom runtime system. The
bytecode is appended to the end of the custom runtime system, so that
it will be automatically executed when the output file (custom
runtime + bytecode) is launched.</p><p>To link in &#X201C;custom runtime&#X201D; mode, execute the <span class="c003">ocamlc</span> command with:
</p><ul class="itemize"><li class="li-itemize">
the <span class="c003">-custom</span> option;
</li><li class="li-itemize">the names of the desired OCaml object files (<span class="c003">.cmo</span> and <span class="c003">.cma</span> files) ;
</li><li class="li-itemize">the names of the C object files and libraries (<span class="c003">.o</span> and <span class="c003">.a</span>
files) that implement the required primitives. Under Unix and Windows,
a library named <span class="c003">lib</span><span class="c009">name</span><span class="c003">.a</span> (respectively, <span class="c003">.lib</span>) residing in one of
the standard library directories can also be specified as <span class="c003">-cclib -l</span><span class="c009">name</span>.
</li></ul><p>If you are using the native-code compiler <span class="c003">ocamlopt</span>, the <span class="c003">-custom</span>
flag is not needed, as the final linking phase of <span class="c003">ocamlopt</span> always
builds a standalone executable. To build a mixed OCaml/C executable,
execute the <span class="c003">ocamlopt</span> command with:
</p><ul class="itemize"><li class="li-itemize">
the names of the desired OCaml native object files (<span class="c003">.cmx</span> and
<span class="c003">.cmxa</span> files);
</li><li class="li-itemize">the names of the C object files and libraries (<span class="c003">.o</span>, <span class="c003">.a</span>,
<span class="c003">.so</span> or <span class="c003">.dll</span> files) that implement the required primitives.
</li></ul><p>Starting with Objective Caml 3.00, it is possible to record the
<span class="c003">-custom</span> option as well as the names of C libraries in an OCaml
library file <span class="c003">.cma</span> or <span class="c003">.cmxa</span>. For instance, consider an OCaml library
<span class="c003">mylib.cma</span>, built from the OCaml object files <span class="c003">a.cmo</span> and <span class="c003">b.cmo</span>,
which reference C code in <span class="c003">libmylib.a</span>. If the library is
built as follows:
</p><pre>
        ocamlc -a -o mylib.cma -custom a.cmo b.cmo -cclib -lmylib
</pre><p>
users of the library can simply link with <span class="c003">mylib.cma</span>:
</p><pre>
        ocamlc -o myprog mylib.cma ...
</pre><p>
and the system will automatically add the <span class="c003">-custom</span> and <span class="c003">-cclib -lmylib</span> options, achieving the same effect as
</p><pre>
        ocamlc -o myprog -custom a.cmo b.cmo ... -cclib -lmylib
</pre><p>
The alternative is of course to build the library without extra
options:
</p><pre>
        ocamlc -a -o mylib.cma a.cmo b.cmo
</pre><p>
and then ask users to provide the <span class="c003">-custom</span> and <span class="c003">-cclib -lmylib</span>
options themselves at link-time:
</p><pre>
        ocamlc -o myprog -custom mylib.cma ... -cclib -lmylib
</pre><p>
The former alternative is more convenient for the final users of the
library, however.</p>
<h3 class="subsection" id="sec417">20.1.4&#XA0;&#XA0;Dynamically linking C code with OCaml code</h3>
<p>
<a id="dynlink-c-code"></a></p><p>Starting with Objective Caml 3.03, an alternative to static linking of C code
using the <span class="c003">-custom</span> code is provided. In this mode, the OCaml linker
generates a pure bytecode executable (no embedded custom runtime
system) that simply records the names of dynamically-loaded libraries
containing the C code. The standard OCaml runtime system <span class="c003">ocamlrun</span>
then loads dynamically these libraries, and resolves references to the
required primitives, before executing the bytecode.</p><p>This facility is currently supported and known to work well under
Linux, MacOS&#XA0;X, and Windows. It is supported, but not
fully tested yet, under FreeBSD, Tru64, Solaris and Irix. It is not
supported yet under other Unixes.</p><p>To dynamically link C code with OCaml code, the C code must first be
compiled into a shared library (under Unix) or DLL (under Windows).
This involves 1- compiling the C files with appropriate C compiler
flags for producing position-independent code (when required by the
operating system), and 2- building a
shared library from the resulting object files. The resulting shared
library or DLL file must be installed in a place where <span class="c003">ocamlrun</span> can
find it later at program start-up time (see
section&#XA0;<a href="runtime.html#s-ocamlrun-dllpath">11.3</a>).
Finally (step 3), execute the <span class="c003">ocamlc</span> command with
</p><ul class="itemize"><li class="li-itemize">
the names of the desired OCaml object files (<span class="c003">.cmo</span> and <span class="c003">.cma</span> files) ;
</li><li class="li-itemize">the names of the C shared libraries (<span class="c003">.so</span> or <span class="c003">.dll</span> files) that
implement the required primitives. Under Unix and Windows,
a library named <span class="c003">dll</span><span class="c009">name</span><span class="c003">.so</span> (respectively, <span class="c003">.dll</span>) residing
in one of the standard library directories can also be specified as
<span class="c003">-dllib -l</span><span class="c009">name</span>.
</li></ul><p>
Do <em>not</em> set the <span class="c003">-custom</span> flag, otherwise you&#X2019;re back to static linking
as described in section&#XA0;<a href="#staticlink-c-code">20.1.3</a>.
The <span class="c003">ocamlmklib</span> tool (see section&#XA0;<a href="#s-ocamlmklib">20.13</a>)
automates steps 2 and 3.</p><p>As in the case of static linking, it is possible (and recommended) to
record the names of C libraries in an OCaml <span class="c003">.cma</span> library archive.
Consider again an OCaml library
<span class="c003">mylib.cma</span>, built from the OCaml object files <span class="c003">a.cmo</span> and <span class="c003">b.cmo</span>,
which reference C code in <span class="c003">dllmylib.so</span>. If the library is
built as follows:
</p><pre>
        ocamlc -a -o mylib.cma a.cmo b.cmo -dllib -lmylib
</pre><p>
users of the library can simply link with <span class="c003">mylib.cma</span>:
</p><pre>
        ocamlc -o myprog mylib.cma ...
</pre><p>
and the system will automatically add the <span class="c003">-dllib -lmylib</span> option,
achieving the same effect as
</p><pre>
        ocamlc -o myprog a.cmo b.cmo ... -dllib -lmylib
</pre><p>
Using this mechanism, users of the library <span class="c003">mylib.cma</span> do not need to
known that it references C code, nor whether this C code must be
statically linked (using <span class="c003">-custom</span>) or dynamically linked.</p>
<h3 class="subsection" id="sec418">20.1.5&#XA0;&#XA0;Choosing between static linking and dynamic linking</h3>
<p>After having described two different ways of linking C code with OCaml
code, we now review the pros and cons of each, to help developers of
mixed OCaml/C libraries decide.</p><p>The main advantage of dynamic linking is that it preserves the
platform-independence of bytecode executables. That is, the bytecode
executable contains no machine code, and can therefore be compiled on
platform <span class="c009">A</span> and executed on other platforms <span class="c009">B</span>, <span class="c009">C</span>, &#X2026;, as long
as the required shared libraries are available on all these
platforms. In contrast, executables generated by <span class="c003">ocamlc -custom</span> run
only on the platform on which they were created, because they embark a
custom-tailored runtime system specific to that platform. In
addition, dynamic linking results in smaller executables.</p><p>Another advantage of dynamic linking is that the final users of the
library do not need to have a C compiler, C linker, and C runtime
libraries installed on their machines. This is no big deal under
Unix and Cygwin, but many Windows users are reluctant to install
Microsoft Visual C just to be able to do <span class="c003">ocamlc -custom</span>.</p><p>There are two drawbacks to dynamic linking. The first is that the
resulting executable is not stand-alone: it requires the shared
libraries, as well as <span class="c003">ocamlrun</span>, to be installed on the machine
executing the code. If you wish to distribute a stand-alone
executable, it is better to link it statically, using <span class="c003">ocamlc -custom -ccopt -static</span> or <span class="c003">ocamlopt -ccopt -static</span>. Dynamic linking also
raises the &#X201C;DLL hell&#X201D; problem: some care must be taken to ensure
that the right versions of the shared libraries are found at start-up
time.</p><p>The second drawback of dynamic linking is that it complicates the
construction of the library. The C compiler and linker flags to
compile to position-independent code and build a shared library vary
wildly between different Unix systems. Also, dynamic linking is not
supported on all Unix systems, requiring a fall-back case to static
linking in the Makefile for the library. The <span class="c003">ocamlmklib</span> command
(see section&#XA0;<a href="#s-ocamlmklib">20.13</a>) tries to hide some of these system
dependencies.</p><p>In conclusion: dynamic linking is highly recommended under the native
Windows port, because there are no portability problems and it is much
more convenient for the end users. Under Unix, dynamic linking should
be considered for mature, frequently used libraries because it
enhances platform-independence of bytecode executables. For new or
rarely-used libraries, static linking is much simpler to set up in a
portable way.</p>
<h3 class="subsection" id="sec419">20.1.6&#XA0;&#XA0;Building standalone custom runtime systems</h3>
<p>
<a id="s:custom-runtime"></a></p><p>It is sometimes inconvenient to build a custom runtime system each
time OCaml code is linked with C libraries, like <span class="c003">ocamlc -custom</span> does.
For one thing, the building of the runtime system is slow on some
systems (that have bad linkers or slow remote file systems); for
another thing, the platform-independence of bytecode files is lost,
forcing to perform one <span class="c003">ocamlc -custom</span> link per platform of interest.</p><p>An alternative to <span class="c003">ocamlc -custom</span> is to build separately a custom
runtime system integrating the desired C libraries, then generate
&#X201C;pure&#X201D; bytecode executables (not containing their own runtime
system) that can run on this custom runtime. This is achieved by the
<span class="c003">-make-runtime</span> and <span class="c003">-use-runtime</span> flags to <span class="c003">ocamlc</span>. For example,
to build a custom runtime system integrating the C parts of the
&#X201C;Unix&#X201D; and &#X201C;Threads&#X201D; libraries, do:
</p><pre>        ocamlc -make-runtime -o /home/me/ocamlunixrun unix.cma threads.cma
</pre><p>To generate a bytecode executable that runs on this runtime system,
do:
</p><pre>
        ocamlc -use-runtime /home/me/ocamlunixrun -o myprog \
                unix.cma threads.cma <span class="c009">your .cmo and .cma files</span>
</pre><p>
The bytecode executable <span class="c003">myprog</span> can then be launched as usual:
<span class="c003">myprog</span> <span class="c009">args</span> or <span class="c003">/home/me/ocamlunixrun myprog</span> <span class="c009">args</span>.</p><p>Notice that the bytecode libraries <span class="c003">unix.cma</span> and <span class="c003">threads.cma</span> must
be given twice: when building the runtime system (so that <span class="c003">ocamlc</span>
knows which C primitives are required) and also when building the
bytecode executable (so that the bytecode from <span class="c003">unix.cma</span> and
<span class="c003">threads.cma</span> is actually linked in).</p>
<h2 class="section" id="sec420">20.2&#XA0;&#XA0;The <span class="c003">value</span> type</h2>
<p>All OCaml objects are represented by the C type <span class="c003">value</span>,
defined in the include file <span class="c003">caml/mlvalues.h</span>, along with macros to
manipulate values of that type. An object of type <span class="c003">value</span> is either:
</p><ul class="itemize"><li class="li-itemize">
an unboxed integer;
</li><li class="li-itemize">a pointer to a block inside the heap (such as the blocks
allocated through one of the <code>caml_alloc_*</code> functions below);
</li><li class="li-itemize">a pointer to an object outside the heap (e.g., a pointer to a block
allocated by <span class="c003">malloc</span>, or to a C variable).
</li></ul>
<h3 class="subsection" id="sec421">20.2.1&#XA0;&#XA0;Integer values</h3>
<p>Integer values encode 63-bit signed integers (31-bit on 32-bit
architectures). They are unboxed (unallocated).</p>
<h3 class="subsection" id="sec422">20.2.2&#XA0;&#XA0;Blocks</h3>
<p>Blocks in the heap are garbage-collected, and therefore have strict
structure constraints. Each block includes a header containing the
size of the block (in words), and the tag of the block.
The tag governs how the contents of the blocks are structured. A tag
lower than <span class="c003">No_scan_tag</span> indicates a structured block, containing
well-formed values, which is recursively traversed by the garbage
collector. A tag greater than or equal to <span class="c003">No_scan_tag</span> indicates a
raw block, whose contents are not scanned by the garbage collector.
For the benefit of ad-hoc polymorphic primitives such as equality and
structured input-output, structured and raw blocks are further
classified according to their tags as follows:
</p><div class="center"><table class="c000 cellpadding1" border=1><tr><td class="c014"><span class="c013">Tag</span></td><td class="c014"><span class="c013">Contents of the block</span> </td></tr>
<tr><td class="c022">
0 to <span class="c003">No_scan_tag</span>&#X2212;1</td><td class="c021">A structured block (an array of
OCaml objects). Each field is a <span class="c003">value</span>. </td></tr>
<tr><td class="c022"><span class="c003">Closure_tag</span></td><td class="c021">A closure representing a functional value. The first
word is a pointer to a piece of code, the remaining words are
<span class="c003">value</span> containing the environment. </td></tr>
<tr><td class="c022"><span class="c003">String_tag</span></td><td class="c021">A character string or a byte sequence. </td></tr>
<tr><td class="c022"><span class="c003">Double_tag</span></td><td class="c021">A double-precision floating-point number. </td></tr>
<tr><td class="c022"><span class="c003">Double_array_tag</span></td><td class="c021">An array or record of double-precision
floating-point numbers. </td></tr>
<tr><td class="c022"><span class="c003">Abstract_tag</span></td><td class="c021">A block representing an abstract datatype. </td></tr>
<tr><td class="c022"><span class="c003">Custom_tag</span></td><td class="c021">A block representing an abstract datatype
with user-defined finalization, comparison, hashing,
serialization and deserialization functions atttached. </td></tr>
</table></div>
<h3 class="subsection" id="sec423">20.2.3&#XA0;&#XA0;Pointers outside the heap</h3>
<p>Any word-aligned pointer to an address outside the heap can be safely
cast to and from the type <span class="c003">value</span>. This includes pointers returned by
<span class="c003">malloc</span>, and pointers to C variables (of size at least one word)
obtained with the <code>&amp;</code> operator.
</p><p>Caution: if a pointer returned by <span class="c003">malloc</span> is cast to the type <span class="c003">value</span>
and returned to OCaml, explicit deallocation of the pointer using
<span class="c003">free</span> is potentially dangerous, because the pointer may still be
accessible from the OCaml world. Worse, the memory space deallocated
by <span class="c003">free</span> can later be reallocated as part of the OCaml heap; the
pointer, formerly pointing outside the OCaml heap, now points inside
the OCaml heap, and this can crash the garbage collector. To avoid
these problems, it is preferable to wrap the pointer in a OCaml block
with tag <span class="c003">Abstract_tag</span> or <span class="c003">Custom_tag</span>.</p>
<h2 class="section" id="sec424">20.3&#XA0;&#XA0;Representation of OCaml data types</h2>
<p>This section describes how OCaml data types are encoded in the
<span class="c003">value</span> type.</p>
<h3 class="subsection" id="sec425">20.3.1&#XA0;&#XA0;Atomic types</h3>
<div class="center"><table class="c000 cellpadding1" border=1><tr><td class="c014"><span class="c013">OCaml type</span></td><td class="c014"><span class="c013">Encoding</span> </td></tr>
<tr><td class="c016">
<span class="c003">int</span></td><td class="c016">Unboxed integer values. </td></tr>
<tr><td class="c016"><span class="c003">char</span></td><td class="c016">Unboxed integer values (ASCII code). </td></tr>
<tr><td class="c016"><span class="c003">float</span></td><td class="c016">Blocks with tag <span class="c003">Double_tag</span>. </td></tr>
<tr><td class="c016"><span class="c003">bytes</span></td><td class="c016">Blocks with tag <span class="c003">String_tag</span>. </td></tr>
<tr><td class="c016"><span class="c003">string</span></td><td class="c016">Blocks with tag <span class="c003">String_tag</span>. </td></tr>
<tr><td class="c016"><span class="c003">int32</span></td><td class="c016">Blocks with tag <span class="c003">Custom_tag</span>. </td></tr>
<tr><td class="c016"><span class="c003">int64</span></td><td class="c016">Blocks with tag <span class="c003">Custom_tag</span>. </td></tr>
<tr><td class="c016"><span class="c003">nativeint</span></td><td class="c016">Blocks with tag <span class="c003">Custom_tag</span>. </td></tr>
</table></div>
<h3 class="subsection" id="sec426">20.3.2&#XA0;&#XA0;Tuples and records</h3>
<p>
<a id="ss:tuples-and-records"></a></p><p>Tuples are represented by pointers to blocks, with tag&#XA0;0.</p><p>Records are also represented by zero-tagged blocks. The ordering of
labels in the record type declaration determines the layout of
the record fields: the value associated to the label
declared first is stored in field&#XA0;0 of the block, the value associated
to the second label goes in field&#XA0;1, and so on.</p><p>As an optimization, records whose fields all have static type <span class="c003">float</span>
are represented as arrays of floating-point numbers, with tag
<span class="c003">Double_array_tag</span>. (See the section below on arrays.)</p><p>As another optimization, unboxable record types are represented
specially; unboxable record types are the immutable record types that
have only one field. An unboxable type will be represented in one of
two ways: boxed or unboxed. Boxed record types are represented as
described above (by a block with tag 0 or <span class="c003">Double_array_tag</span>). An
unboxed record type is represented directly by the value of its field
(i.e. there is no block to represent the record itself).</p><p>The representation is chosen according to the following, in decreasing
order of priority:
</p><ul class="itemize"><li class="li-itemize">
An attribute (<span class="c003">[@@boxed]</span> or <span class="c003">[@@unboxed]</span>) on the type declaration.
</li><li class="li-itemize">A compiler option (<span class="c003">-unboxed-types</span> or <span class="c003">-no-unboxed-types</span>).
</li><li class="li-itemize">The default representation. In the present version of OCaml, the
default is the boxed representation.
</li></ul>
<h3 class="subsection" id="sec427">20.3.3&#XA0;&#XA0;Arrays</h3>
<p>Arrays of integers and pointers are represented like tuples,
that is, as pointers to blocks tagged&#XA0;0. They are accessed with the
<span class="c003">Field</span> macro for reading and the <span class="c003">caml_modify</span> function for writing.</p><p>Arrays of floating-point numbers (type <span class="c003">float array</span>)
have a special, unboxed, more efficient representation.
These arrays are represented by pointers to blocks with tag
<span class="c003">Double_array_tag</span>. They should be accessed with the <span class="c003">Double_field</span>
and <span class="c003">Store_double_field</span> macros.</p>
<h3 class="subsection" id="sec428">20.3.4&#XA0;&#XA0;Concrete data types</h3>
<p>Constructed terms are represented either by unboxed integers (for
constant constructors) or by blocks whose tag encode the constructor
(for non-constant constructors). The constant constructors and the
non-constant constructors for a given concrete type are numbered
separately, starting from 0, in the order in which they appear in the
concrete type declaration. A constant constructor is represented by
the unboxed integer equal to its constructor number. A non-constant
constructor declared with <span class="c009">n</span> arguments is represented by
a block of size <span class="c009">n</span>, tagged with the constructor number; the <span class="c009">n</span>
fields contain its arguments. Example:</p><div class="center"><table class="c000 cellpadding1" border=1><tr><td class="c014"><span class="c013">Constructed term</span></td><td class="c014"><span class="c013">Representation</span> </td></tr>
<tr><td class="c022">
<span class="c003">()</span></td><td class="c021"><span class="c003">Val_int(0)</span> </td></tr>
<tr><td class="c022"><span class="c003">false</span></td><td class="c021"><span class="c003">Val_int(0)</span> </td></tr>
<tr><td class="c022"><span class="c003">true</span></td><td class="c021"><span class="c003">Val_int(1)</span> </td></tr>
<tr><td class="c022"><span class="c003">[]</span></td><td class="c021"><span class="c003">Val_int(0)</span> </td></tr>
<tr><td class="c022"><span class="c003">h::t</span></td><td class="c021">Block with size = 2 and tag = 0; first field
contains <span class="c003">h</span>, second field <span class="c003">t</span>. </td></tr>
</table></div><p>As a convenience, <span class="c003">caml/mlvalues.h</span> defines the macros <span class="c003">Val_unit</span>,
<span class="c003">Val_false</span> and <span class="c003">Val_true</span> to refer to <span class="c003">()</span>, <span class="c003">false</span> and <span class="c003">true</span>.</p><p>The following example illustrates the assignment of
integers and block tags to constructors:
</p><pre>type t =
  | A             (* First constant constructor -&gt; integer "Val_int(0)" *)
  | B of string   (* First non-constant constructor -&gt; block with tag 0 *)
  | C             (* Second constant constructor -&gt; integer "Val_int(1)" *)
  | D of bool     (* Second non-constant constructor -&gt; block with tag 1 *)
  | E of t * t    (* Third non-constant constructor -&gt; block with tag 2 *)
</pre><p>As an optimization, unboxable concrete data types are represented
specially; a concrete data type is unboxable if it has exactly one
constructor and this constructor has exactly one argument. Unboxable
concrete data types are represented in the same ways as unboxable
record types: see the description in
section&#XA0;<a href="#ss%3Atuples-and-records">20.3.2</a>.</p>
<h3 class="subsection" id="sec429">20.3.5&#XA0;&#XA0;Objects</h3>
<p>Objects are represented as blocks with tag <span class="c003">Object_tag</span>. The first
field of the block refers to the object&#X2019;s class and associated method
suite, in a format that cannot easily be exploited from C. The second
field contains a unique object ID, used for comparisons. The remaining
fields of the object contain the values of the instance variables of
the object. It is unsafe to access directly instance variables, as the
type system provides no guarantee about the instance variables
contained by an object.
</p><p>One may extract a public method from an object using the C function
<span class="c003">caml_get_public_method</span> (declared in <span class="c003">&lt;caml/mlvalues.h&gt;</span>.)
Since public method tags are hashed in the same way as variant tags,
and methods are functions taking self as first argument, if you want
to do the method call <span class="c003">foo#bar</span> from the C side, you should call:
</p><pre>  callback(caml_get_public_method(foo, hash_variant("bar")), foo);
</pre>
<h3 class="subsection" id="sec430">20.3.6&#XA0;&#XA0;Polymorphic variants</h3>
<p>Like constructed terms, polymorphic variant values are represented either
as integers (for polymorphic variants without argument), or as blocks
(for polymorphic variants with an argument). Unlike constructed
terms, variant constructors are not numbered starting from 0, but
identified by a hash value (an OCaml integer), as computed by the C function
<span class="c003">hash_variant</span> (declared in <span class="c003">&lt;caml/mlvalues.h&gt;</span>):
the hash value for a variant constructor named, say, <span class="c003">VConstr</span>
is <span class="c003">hash_variant("VConstr")</span>.</p><p>The variant value <span class="c003">`VConstr</span> is represented by
<span class="c003">hash_variant("VConstr")</span>. The variant value <span class="c003">`VConstr(</span><span class="c009">v</span><span class="c003">)</span> is
represented by a block of size 2 and tag 0, with field number 0
containing <span class="c003">hash_variant("VConstr")</span> and field number 1 containing
<span class="c009">v</span>.</p><p>Unlike constructed values, polymorphic variant values taking several
arguments are not flattened.
That is, <span class="c003">`VConstr(</span><span class="c009">v</span><span class="c003">, </span><span class="c009">w</span><span class="c003">)</span> is represented by a block
of size 2, whose field number 1 contains the representation of the
pair <span class="c003">(</span><span class="c009">v</span><span class="c003">, </span><span class="c009">w</span><span class="c003">)</span>, rather than a block of size 3
containing <span class="c009">v</span> and <span class="c009">w</span> in fields 1 and 2.</p>
<h2 class="section" id="sec431">20.4&#XA0;&#XA0;Operations on values</h2>
<h3 class="subsection" id="sec432">20.4.1&#XA0;&#XA0;Kind tests</h3>
<ul class="itemize"><li class="li-itemize">
<span class="c003">Is_long(</span><span class="c009">v</span><span class="c003">)</span> is true if value <span class="c009">v</span> is an immediate integer,
false otherwise
</li><li class="li-itemize"><span class="c003">Is_block(</span><span class="c009">v</span><span class="c003">)</span> is true if value <span class="c009">v</span> is a pointer to a block,
and false if it is an immediate integer.
</li></ul>
<h3 class="subsection" id="sec433">20.4.2&#XA0;&#XA0;Operations on integers</h3>
<ul class="itemize"><li class="li-itemize">
<span class="c003">Val_long(</span><span class="c009">l</span><span class="c003">)</span> returns the value encoding the <span class="c003">long int</span> <span class="c009">l</span>.
</li><li class="li-itemize"><span class="c003">Long_val(</span><span class="c009">v</span><span class="c003">)</span> returns the <span class="c003">long int</span> encoded in value <span class="c009">v</span>.
</li><li class="li-itemize"><span class="c003">Val_int(</span><span class="c009">i</span><span class="c003">)</span> returns the value encoding the <span class="c003">int</span> <span class="c009">i</span>.
</li><li class="li-itemize"><span class="c003">Int_val(</span><span class="c009">v</span><span class="c003">)</span> returns the <span class="c003">int</span> encoded in value <span class="c009">v</span>.
</li><li class="li-itemize"><span class="c003">Val_bool(</span><span class="c009">x</span><span class="c003">)</span> returns the OCaml boolean representing the
truth value of the C integer <span class="c009">x</span>.
</li><li class="li-itemize"><span class="c003">Bool_val(</span><span class="c009">v</span><span class="c003">)</span> returns 0 if <span class="c009">v</span> is the OCaml boolean
<span class="c003">false</span>, 1 if <span class="c009">v</span> is <span class="c003">true</span>.
</li><li class="li-itemize"><span class="c003">Val_true</span>, <span class="c003">Val_false</span> represent the OCaml booleans <span class="c003">true</span> and <span class="c003">false</span>.
</li></ul>
<h3 class="subsection" id="sec434">20.4.3&#XA0;&#XA0;Accessing blocks</h3>
<ul class="itemize"><li class="li-itemize">
<span class="c003">Wosize_val(</span><span class="c009">v</span><span class="c003">)</span> returns the size of the block <span class="c009">v</span>, in words,
excluding the header.
</li><li class="li-itemize"><span class="c003">Tag_val(</span><span class="c009">v</span><span class="c003">)</span> returns the tag of the block <span class="c009">v</span>.
</li><li class="li-itemize"><span class="c003">Field(</span><span class="c009">v</span><span class="c003">, </span><span class="c009">n</span><span class="c003">)</span> returns the value contained in the
<span class="c009">n</span><sup><span class="c008">th</span></sup> field of the structured block <span class="c009">v</span>. Fields are numbered from 0 to
<span class="c003">Wosize_val</span>(<span class="c009">v</span>)&#X2212;1.
</li><li class="li-itemize"><span class="c003">Store_field(</span><span class="c009">b</span><span class="c003">, </span><span class="c009">n</span><span class="c003">, </span><span class="c009">v</span><span class="c003">)</span> stores the value
<span class="c009">v</span> in the field number <span class="c009">n</span> of value <span class="c009">b</span>, which must be a
structured block.
</li><li class="li-itemize"><span class="c003">Code_val(</span><span class="c009">v</span><span class="c003">)</span> returns the code part of the closure <span class="c009">v</span>.
</li><li class="li-itemize"><span class="c003">caml_string_length(</span><span class="c009">v</span><span class="c003">)</span> returns the length (number of bytes)
of the string or byte sequence <span class="c009">v</span>.
</li><li class="li-itemize"><span class="c003">Byte(</span><span class="c009">v</span><span class="c003">, </span><span class="c009">n</span><span class="c003">)</span> returns the <span class="c009">n</span><sup><span class="c008">th</span></sup> byte of the string
or byte sequence <span class="c009">v</span>, with type <span class="c003">char</span>. Bytes are numbered from 0 to
<span class="c003">string_length</span>(<span class="c009">v</span>)&#X2212;1.
</li><li class="li-itemize"><span class="c003">Byte_u(</span><span class="c009">v</span><span class="c003">, </span><span class="c009">n</span><span class="c003">)</span> returns the <span class="c009">n</span><sup><span class="c008">th</span></sup> byte of the string
or byte sequence <span class="c009">v</span>, with type <span class="c003">unsigned char</span>. Bytes are
numbered from 0 to <span class="c003">string_length</span>(<span class="c009">v</span>)&#X2212;1.
</li><li class="li-itemize"><span class="c003">String_val(</span><span class="c009">v</span><span class="c003">)</span> returns a pointer to the first byte of the string
<span class="c009">v</span>, with type <span class="c003">char *</span> or, when OCaml is configured with
<span class="c003">-force-safe-string</span>, with type <span class="c003">const char *</span>.
This pointer is a valid C string: there is a null byte after the last
byte in the string. However, OCaml strings can contain embedded null bytes,
which will confuse the usual C functions over strings.
</li><li class="li-itemize"><span class="c003">Bytes_val(</span><span class="c009">v</span><span class="c003">)</span> returns a pointer to the first byte of the
byte sequence <span class="c009">v</span>, with type <span class="c003">unsigned char *</span>.
</li><li class="li-itemize"><span class="c003">Double_val(</span><span class="c009">v</span><span class="c003">)</span> returns the floating-point number contained in
value <span class="c009">v</span>, with type <span class="c003">double</span>.
</li><li class="li-itemize"><span class="c003">Double_field(</span><span class="c009">v</span><span class="c003">, </span><span class="c009">n</span><span class="c003">)</span> returns
the <span class="c009">n</span><sup><span class="c008">th</span></sup> element of the array of floating-point numbers <span class="c009">v</span> (a
block tagged <span class="c003">Double_array_tag</span>).
</li><li class="li-itemize"><span class="c003">Store_double_field(</span><span class="c009">v</span><span class="c003">, </span><span class="c009">n</span><span class="c003">, </span><span class="c009">d</span><span class="c003">)</span> stores the double precision floating-point number <span class="c009">d</span>
in the <span class="c009">n</span><sup><span class="c008">th</span></sup> element of the array of floating-point numbers <span class="c009">v</span>.
</li><li class="li-itemize"><span class="c003">Data_custom_val(</span><span class="c009">v</span><span class="c003">)</span> returns a pointer to the data part
of the custom block <span class="c009">v</span>. This pointer has type <span class="c003">void *</span> and must
be cast to the type of the data contained in the custom block.
</li><li class="li-itemize"><span class="c003">Int32_val(</span><span class="c009">v</span><span class="c003">)</span> returns the 32-bit integer contained
in the <span class="c003">int32</span> <span class="c009">v</span>.
</li><li class="li-itemize"><span class="c003">Int64_val(</span><span class="c009">v</span><span class="c003">)</span> returns the 64-bit integer contained
in the <span class="c003">int64</span> <span class="c009">v</span>.
</li><li class="li-itemize"><span class="c003">Nativeint_val(</span><span class="c009">v</span><span class="c003">)</span> returns the long integer contained
in the <span class="c003">nativeint</span> <span class="c009">v</span>.
</li><li class="li-itemize"><span class="c003">caml_field_unboxed(</span><span class="c009">v</span><span class="c003">)</span> returns the value of the field
of a value <span class="c009">v</span> of any unboxed type (record or concrete data type).
</li><li class="li-itemize"><span class="c003">caml_field_boxed(</span><span class="c009">v</span><span class="c003">)</span> returns the value of the field
of a value <span class="c009">v</span> of any boxed type (record or concrete data type).
</li><li class="li-itemize"><span class="c003">caml_field_unboxable(</span><span class="c009">v</span><span class="c003">)</span> calls either
<span class="c003">caml_field_unboxed</span> or <span class="c003">caml_field_boxed</span> according to the default
representation of unboxable types in the current version of OCaml.
</li></ul><p>
The expressions <span class="c003">Field(</span><span class="c009">v</span><span class="c003">, </span><span class="c009">n</span><span class="c003">)</span>,
<span class="c003">Byte(</span><span class="c009">v</span><span class="c003">, </span><span class="c009">n</span><span class="c003">)</span> and
<span class="c003">Byte_u(</span><span class="c009">v</span><span class="c003">, </span><span class="c009">n</span><span class="c003">)</span>
are valid l-values. Hence, they can be assigned to, resulting in an
in-place modification of value <span class="c009">v</span>.
Assigning directly to <span class="c003">Field(</span><span class="c009">v</span><span class="c003">, </span><span class="c009">n</span><span class="c003">)</span> must
be done with care to avoid confusing the garbage collector (see
below).</p>
<h3 class="subsection" id="sec435">20.4.4&#XA0;&#XA0;Allocating blocks</h3>
<h4 class="subsubsection" id="sec436">Simple interface</h4>
<ul class="itemize"><li class="li-itemize">
<span class="c003">Atom(</span><span class="c009">t</span><span class="c003">)</span> returns an &#X201C;atom&#X201D; (zero-sized block) with tag <span class="c009">t</span>.
Zero-sized blocks are preallocated outside of the heap. It is
incorrect to try and allocate a zero-sized block using the functions below.
For instance, <span class="c003">Atom(0)</span> represents the empty array.
</li><li class="li-itemize"><span class="c003">caml_alloc(</span><span class="c009">n</span><span class="c003">, </span><span class="c009">t</span><span class="c003">)</span> returns a fresh block of size <span class="c009">n</span>
with tag <span class="c009">t</span>. If <span class="c009">t</span> is less than <span class="c003">No_scan_tag</span>, then the
fields of the block are initialized with a valid value in order to
satisfy the GC constraints.
</li><li class="li-itemize"><span class="c003">caml_alloc_tuple(</span><span class="c009">n</span><span class="c003">)</span> returns a fresh block of size
<span class="c009">n</span> words, with tag 0.
</li><li class="li-itemize"><span class="c003">caml_alloc_string(</span><span class="c009">n</span><span class="c003">)</span> returns a byte sequence (or string) value of
length <span class="c009">n</span> bytes. The sequence initially contains uninitialized bytes.
</li><li class="li-itemize"><span class="c003">caml_alloc_initialized_string(</span><span class="c009">n</span><span class="c003">, </span><span class="c009">p</span><span class="c003">)</span> returns a byte sequence
(or string) value of length <span class="c009">n</span> bytes. The value is initialized from the
<span class="c009">n</span> bytes starting at address <span class="c009">p</span>.
</li><li class="li-itemize"><span class="c003">caml_copy_string(</span><span class="c009">s</span><span class="c003">)</span> returns a string or byte sequence value
containing a copy of the null-terminated C string <span class="c009">s</span> (a <span class="c003">char *</span>).
</li><li class="li-itemize"><span class="c003">caml_copy_double(</span><span class="c009">d</span><span class="c003">)</span> returns a floating-point value initialized
with the <span class="c003">double</span> <span class="c009">d</span>.
</li><li class="li-itemize"><span class="c003">caml_copy_int32(</span><span class="c009">i</span><span class="c003">)</span>, <span class="c003">caml_copy_int64(</span><span class="c009">i</span><span class="c003">)</span> and
<span class="c003">caml_copy_nativeint(</span><span class="c009">i</span><span class="c003">)</span> return a value of OCaml type <span class="c003">int32</span>,
<span class="c003">int64</span> and <span class="c003">nativeint</span>, respectively, initialized with the integer
<span class="c009">i</span>.
</li><li class="li-itemize"><span class="c003">caml_alloc_array(</span><span class="c009">f</span><span class="c003">, </span><span class="c009">a</span><span class="c003">)</span> allocates an array of values, calling
function <span class="c009">f</span> over each element of the input array <span class="c009">a</span> to transform it
into a value. The array <span class="c009">a</span> is an array of pointers terminated by the
null pointer. The function <span class="c009">f</span> receives each pointer as argument, and
returns a value. The zero-tagged block returned by
<span class="c003">alloc_array(</span><span class="c009">f</span><span class="c003">, </span><span class="c009">a</span><span class="c003">)</span> is filled with the values returned by the
successive calls to <span class="c009">f</span>. (This function must not be used to build
an array of floating-point numbers.)
</li><li class="li-itemize"><span class="c003">caml_copy_string_array(</span><span class="c009">p</span><span class="c003">)</span> allocates an array of strings or byte
sequences, copied from the pointer to a string array <span class="c009">p</span>
(a <span class="c003">char **</span>). <span class="c009">p</span> must be NULL-terminated.
</li><li class="li-itemize"><span class="c003">caml_alloc_float_array(</span><span class="c009">n</span><span class="c003">)</span> allocates an array of floating point
numbers of size <span class="c009">n</span>. The array initially contains uninitialized values.
</li><li class="li-itemize"><span class="c003">caml_alloc_unboxed(</span><span class="c009">v</span><span class="c003">)</span> returns the value (of any unboxed
type) whose field is the value <span class="c009">v</span>.
</li><li class="li-itemize"><span class="c003">caml_alloc_boxed(</span><span class="c009">v</span><span class="c003">)</span> allocates and returns a value (of
any boxed type) whose field is the value <span class="c009">v</span>.
</li><li class="li-itemize"><span class="c003">caml_alloc_unboxable(</span><span class="c009">v</span><span class="c003">)</span> calls either
<span class="c003">caml_alloc_unboxed</span> or <span class="c003">caml_alloc_boxed</span> according to the default
representation of unboxable types in the current version of OCaml.
</li></ul>
<h4 class="subsubsection" id="sec437">Low-level interface</h4>
<p>The following functions are slightly more efficient than <span class="c003">caml_alloc</span>, but
also much more difficult to use.</p><p>From the standpoint of the allocation functions, blocks are divided
according to their size as zero-sized blocks, small blocks (with size
less than or equal to <code>Max_young_wosize</code>), and large blocks (with
size greater than <code>Max_young_wosize</code>). The constant
<code>Max_young_wosize</code> is declared in the include file <span class="c003">mlvalues.h</span>. It
is guaranteed to be at least 64 (words), so that any block with
constant size less than or equal to 64 can be assumed to be small. For
blocks whose size is computed at run-time, the size must be compared
against <code>Max_young_wosize</code> to determine the correct allocation procedure.</p><ul class="itemize"><li class="li-itemize">
<span class="c003">caml_alloc_small(</span><span class="c009">n</span><span class="c003">, </span><span class="c009">t</span><span class="c003">)</span> returns a fresh small block of size
<span class="c009">n</span> &#X2264; <span class="c003">Max_young_wosize</span> words, with tag <span class="c009">t</span>.
If this block is a structured block (i.e. if <span class="c009">t</span> &lt; <span class="c003">No_scan_tag</span>), then
the fields of the block (initially containing garbage) must be initialized
with legal values (using direct assignment to the fields of the block)
before the next allocation.
</li><li class="li-itemize"><span class="c003">caml_alloc_shr(</span><span class="c009">n</span><span class="c003">, </span><span class="c009">t</span><span class="c003">)</span> returns a fresh block of size
<span class="c009">n</span>, with tag <span class="c009">t</span>.
The size of the block can be greater than <code>Max_young_wosize</code>. (It
can also be smaller, but in this case it is more efficient to call
<span class="c003">caml_alloc_small</span> instead of <span class="c003">caml_alloc_shr</span>.)
If this block is a structured block (i.e. if <span class="c009">t</span> &lt; <span class="c003">No_scan_tag</span>), then
the fields of the block (initially containing garbage) must be initialized
with legal values (using the <span class="c003">caml_initialize</span> function described below)
before the next allocation.
</li></ul>
<h3 class="subsection" id="sec438">20.4.5&#XA0;&#XA0;Raising exceptions</h3>
<p> <a id="s:c-exceptions"></a></p><p>Two functions are provided to raise two standard exceptions:
</p><ul class="itemize"><li class="li-itemize">
<span class="c003">caml_failwith(</span><span class="c009">s</span><span class="c003">)</span>, where <span class="c009">s</span> is a null-terminated C string (with
type <code>char *</code>), raises exception <span class="c003">Failure</span> with argument <span class="c009">s</span>.
</li><li class="li-itemize"><span class="c003">caml_invalid_argument(</span><span class="c009">s</span><span class="c003">)</span>, where <span class="c009">s</span> is a null-terminated C
string (with type <code>char *</code>), raises exception <span class="c003">Invalid_argument</span>
with argument <span class="c009">s</span>.
</li></ul><p>Raising arbitrary exceptions from C is more delicate: the
exception identifier is dynamically allocated by the OCaml program, and
therefore must be communicated to the C function using the
registration facility described below in section&#XA0;<a href="#s%3Aregister-exn">20.7.3</a>.
Once the exception identifier is recovered in C, the following
functions actually raise the exception:
</p><ul class="itemize"><li class="li-itemize">
<span class="c003">caml_raise_constant(</span><span class="c009">id</span><span class="c003">)</span> raises the exception <span class="c009">id</span> with
no argument;
</li><li class="li-itemize"><span class="c003">caml_raise_with_arg(</span><span class="c009">id</span><span class="c003">, </span><span class="c009">v</span><span class="c003">)</span> raises the exception
<span class="c009">id</span> with the OCaml value <span class="c009">v</span> as argument;
</li><li class="li-itemize"><span class="c003">caml_raise_with_args(</span><span class="c009">id</span><span class="c003">, </span><span class="c009">n</span><span class="c003">, </span><span class="c009">v</span><span class="c003">)</span>
raises the exception <span class="c009">id</span> with the OCaml values
<span class="c009">v</span><span class="c003">[0]</span>, &#X2026;, <span class="c009">v</span><span class="c003">[</span><span class="c009">n</span><span class="c003">-1]</span> as arguments;
</li><li class="li-itemize"><span class="c003">caml_raise_with_string(</span><span class="c009">id</span><span class="c003">, </span><span class="c009">s</span><span class="c003">)</span>, where <span class="c009">s</span> is a
null-terminated C string, raises the exception <span class="c009">id</span> with a copy of
the C string <span class="c009">s</span> as argument.
</li></ul>
<h2 class="section" id="sec439">20.5&#XA0;&#XA0;Living in harmony with the garbage collector</h2>
<p>Unused blocks in the heap are automatically reclaimed by the garbage
collector. This requires some cooperation from C code that
manipulates heap-allocated blocks.</p>
<h3 class="subsection" id="sec440">20.5.1&#XA0;&#XA0;Simple interface</h3>
<p>All the macros described in this section are declared in the
<span class="c003">memory.h</span> header file.</p><div class="theorem"><span class="c013">Rule&#XA0;1</span>&#XA0;&#XA0;<em>
A function that has parameters or local variables of type <span class="c003">value</span> must
begin with a call to one of the <span class="c003">CAMLparam</span> macros and return with
<span class="c003">CAMLreturn</span>, <span class="c003">CAMLreturn0</span>, or <span class="c003">CAMLreturnT</span>. In particular, <span class="c003">CAMLlocal</span>
and <span class="c003">CAMLxparam</span> can only be called </em>after <em><span class="c003">CAMLparam</span>.
</em></div><p>There are six <span class="c003">CAMLparam</span> macros: <span class="c003">CAMLparam0</span> to <span class="c003">CAMLparam5</span>, which
take zero to five arguments respectively. If your function has no more
than 5 parameters of type <span class="c003">value</span>, use the corresponding macros
with these parameters as arguments. If your function has more than 5
parameters of type <span class="c003">value</span>, use <span class="c003">CAMLparam5</span> with five of these
parameters, and use one or more calls to the <span class="c003">CAMLxparam</span> macros for
the remaining parameters (<span class="c003">CAMLxparam1</span> to <span class="c003">CAMLxparam5</span>).</p><p>The macros <span class="c003">CAMLreturn</span>, <span class="c003">CAMLreturn0</span>, and <span class="c003">CAMLreturnT</span> are used to
replace the C
keyword <span class="c003">return</span>. Every occurrence of <span class="c003">return x</span> must be replaced by
<span class="c003">CAMLreturn (x)</span> if <span class="c003">x</span> has type <span class="c003">value</span>, or <span class="c003">CAMLreturnT (t, x)</span>
(where <span class="c003">t</span> is the type of <span class="c003">x</span>); every occurrence of <span class="c003">return</span> without
argument must be
replaced by <span class="c003">CAMLreturn0</span>. If your C function is a procedure (i.e. if
it returns void), you must insert <span class="c003">CAMLreturn0</span> at the end (to replace
C&#X2019;s implicit <span class="c003">return</span>).</p>
<h5 class="paragraph" id="sec441">Note:</h5>
<p> some C compilers give bogus warnings about unused
variables <span class="c003">caml__dummy_xxx</span> at each use of <span class="c003">CAMLparam</span> and
<span class="c003">CAMLlocal</span>. You should ignore them.</p><p> <br>
</p><p>Example:
</p><pre>void foo (value v1, value v2, value v3)
{
  CAMLparam3 (v1, v2, v3);
  ...
  CAMLreturn0;
}
</pre>
<h5 class="paragraph" id="sec442">Note:</h5>
<p> if your function is a primitive with more than 5 arguments
for use with the byte-code runtime, its arguments are not <span class="c003">value</span>s and
must not be declared (they have types <span class="c003">value *</span> and <span class="c003">int</span>).</p><div class="theorem"><span class="c013">Rule&#XA0;2</span>&#XA0;&#XA0;<em>
Local variables of type <span class="c003">value</span> must be declared with one of the
<span class="c003">CAMLlocal</span> macros. Arrays of <span class="c003">value</span>s are declared with
<span class="c003">CAMLlocalN</span>. These macros must be used at the beginning of the
function, not in a nested block.
</em></div><p>The macros <span class="c003">CAMLlocal1</span> to <span class="c003">CAMLlocal5</span> declare and initialize one to
five local variables of type <span class="c003">value</span>. The variable names are given as
arguments to the macros. <span class="c003">CAMLlocalN(</span><span class="c009">x</span><span class="c003">, </span><span class="c009">n</span><span class="c003">)</span> declares
and initializes a local variable of type <span class="c003">value [</span><span class="c009">n</span><span class="c003">]</span>. You can
use several calls to these macros if you have more than 5 local
variables.</p><p>Example:
</p><pre>value bar (value v1, value v2, value v3)
{
  CAMLparam3 (v1, v2, v3);
  CAMLlocal1 (result);
  result = caml_alloc (3, 0);
  ...
  CAMLreturn (result);
}
</pre><div class="theorem"><span class="c013">Rule&#XA0;3</span>&#XA0;&#XA0;<em>
Assignments to the fields of structured blocks must be done with the
<span class="c003">Store_field</span> macro (for normal blocks) or <span class="c003">Store_double_field</span> macro
(for arrays and records of floating-point numbers). Other assignments
must not use <span class="c003">Store_field</span> nor <span class="c003">Store_double_field</span>.
</em></div><p><span class="c003">Store_field (</span><span class="c009">b</span><span class="c003">, </span><span class="c009">n</span><span class="c003">, </span><span class="c009">v</span><span class="c003">)</span> stores the value
<span class="c009">v</span> in the field number <span class="c009">n</span> of value <span class="c009">b</span>, which must be a
block (i.e. <span class="c003">Is_block(</span><span class="c009">b</span><span class="c003">)</span> must be true).</p><p>Example:
</p><pre>value bar (value v1, value v2, value v3)
{
  CAMLparam3 (v1, v2, v3);
  CAMLlocal1 (result);
  result = caml_alloc (3, 0);
  Store_field (result, 0, v1);
  Store_field (result, 1, v2);
  Store_field (result, 2, v3);
  CAMLreturn (result);
}
</pre>
<h5 class="paragraph" id="sec443">Warning:</h5>
<p> The first argument of <span class="c003">Store_field</span> and
<span class="c003">Store_double_field</span> must be a variable declared by <span class="c003">CAMLparam*</span> or
a parameter declared by <span class="c003">CAMLlocal*</span> to ensure that a garbage
collection triggered by the evaluation of the other arguments will not
invalidate the first argument after it is computed.</p>
<h5 class="paragraph" id="sec444">Use with CAMLlocalN:</h5>
<p> Arrays of values declared using
<span class="c003">CAMLlocalN</span> must not be written to using <span class="c003">Store_field</span>.
Use the normal C array syntax instead.</p><div class="theorem"><span class="c013">Rule&#XA0;4</span>&#XA0;&#XA0;<em> Global variables containing values must be registered
with the garbage collector using the <span class="c003">caml_register_global_root</span> function.
</em></div><p>Registration of a global variable <span class="c003">v</span> is achieved by calling
<span class="c003">caml_register_global_root(&amp;v)</span> just before or just after a valid
value is stored in <span class="c003">v</span> for the first time. You must not call any
of the OCaml runtime functions or macros between registering and
storing the value.</p><p>A registered global variable <span class="c003">v</span> can be un-registered by calling
<span class="c003">caml_remove_global_root(&amp;v)</span>.</p><p>If the contents of the global variable <span class="c003">v</span> are seldom modified after
registration, better performance can be achieved by calling
<span class="c003">caml_register_generational_global_root(&amp;v)</span> to register <span class="c003">v</span> (after
its initialization with a valid <span class="c003">value</span>, but before any allocation or
call to the GC functions),
and <span class="c003">caml_remove_generational_global_root(&amp;v)</span> to un-register it. In
this case, you must not modify the value of <span class="c003">v</span> directly, but you must
use <span class="c003">caml_modify_generational_global_root(&amp;v,x)</span> to set it to <span class="c003">x</span>.
The garbage collector takes advantage of the guarantee that <span class="c003">v</span> is not
modified between calls to <span class="c003">caml_modify_generational_global_root</span> to scan it
less often. This improves performance if the
modifications of <span class="c003">v</span> happen less often than minor collections.</p>
<h5 class="paragraph" id="sec445">Note:</h5>
<p> The <span class="c003">CAML</span> macros use identifiers (local variables, type
identifiers, structure tags) that start with <span class="c003">caml__</span>. Do not use any
identifier starting with <span class="c003">caml__</span> in your programs.</p>
<h3 class="subsection" id="sec446">20.5.2&#XA0;&#XA0;Low-level interface</h3>
<p>We now give the GC rules corresponding to the low-level allocation
functions <span class="c003">caml_alloc_small</span> and <span class="c003">caml_alloc_shr</span>. You can ignore those rules
if you stick to the simplified allocation function <span class="c003">caml_alloc</span>.</p><div class="theorem"><span class="c013">Rule&#XA0;5</span>&#XA0;&#XA0;<em> After a structured block (a block with tag less than
<span class="c003">No_scan_tag</span>) is allocated with the low-level functions, all fields
of this block must be filled with well-formed values before the next
allocation operation. If the block has been allocated with
<span class="c003">caml_alloc_small</span>, filling is performed by direct assignment to the fields
of the block:
</em><pre><em>
        Field(<span class="c009">v</span>, <span class="c009">n</span>) = </em><span class="c009">v</span><sub><span class="c009">n</span></sub><em>;
</em></pre><em>
If the block has been allocated with <span class="c003">caml_alloc_shr</span>, filling is performed
through the <span class="c003">caml_initialize</span> function:
</em><pre><em>
        caml_initialize(&amp;Field(<span class="c009">v</span>, <span class="c009">n</span>), </em><span class="c009">v</span><sub><span class="c009">n</span></sub><em>);
</em></pre>
</div><p>The next allocation can trigger a garbage collection. The garbage
collector assumes that all structured blocks contain well-formed
values. Newly created blocks contain random data, which generally do
not represent well-formed values.</p><p>If you really need to allocate before the fields can receive their
final value, first initialize with a constant value (e.g.
<span class="c003">Val_unit</span>), then allocate, then modify the fields with the correct
value (see rule&#XA0;6).</p><div class="theorem"><span class="c013">Rule&#XA0;6</span>&#XA0;&#XA0;<em> Direct assignment to a field of a block, as in
</em><pre><em>
        Field(<span class="c009">v</span>, <span class="c009">n</span>) = <span class="c009">w</span>;
</em></pre><em>
is safe only if <span class="c009">v</span> is a block newly allocated by <span class="c003">caml_alloc_small</span>;
that is, if no allocation took place between the
allocation of <span class="c009">v</span> and the assignment to the field. In all other cases,
never assign directly. If the block has just been allocated by <span class="c003">caml_alloc_shr</span>,
use <span class="c003">caml_initialize</span> to assign a value to a field for the first time:
</em><pre><em>
        caml_initialize(&amp;Field(<span class="c009">v</span>, <span class="c009">n</span>), <span class="c009">w</span>);
</em></pre><em>
Otherwise, you are updating a field that previously contained a
well-formed value; then, call the <span class="c003">caml_modify</span> function:
</em><pre><em>
        caml_modify(&amp;Field(<span class="c009">v</span>, <span class="c009">n</span>), <span class="c009">w</span>);
</em></pre>
</div><p>To illustrate the rules above, here is a C function that builds and
returns a list containing the two integers given as parameters.
First, we write it using the simplified allocation functions:
</p><pre>value alloc_list_int(int i1, int i2)
{
  CAMLparam0 ();
  CAMLlocal2 (result, r);

  r = caml_alloc(2, 0);                   /* Allocate a cons cell */
  Store_field(r, 0, Val_int(i2));         /* car = the integer i2 */
  Store_field(r, 1, Val_int(0));          /* cdr = the empty list [] */
  result = caml_alloc(2, 0);              /* Allocate the other cons cell */
  Store_field(result, 0, Val_int(i1));    /* car = the integer i1 */
  Store_field(result, 1, r);              /* cdr = the first cons cell */
  CAMLreturn (result);
}
</pre><p>Here, the registering of <span class="c003">result</span> is not strictly needed, because no
allocation takes place after it gets its value, but it&#X2019;s easier and
safer to simply register all the local variables that have type <span class="c003">value</span>.</p><p>Here is the same function written using the low-level allocation
functions. We notice that the cons cells are small blocks and can be
allocated with <span class="c003">caml_alloc_small</span>, and filled by direct assignments on
their fields.
</p><pre>value alloc_list_int(int i1, int i2)
{
  CAMLparam0 ();
  CAMLlocal2 (result, r);

  r = caml_alloc_small(2, 0);             /* Allocate a cons cell */
  Field(r, 0) = Val_int(i2);              /* car = the integer i2 */
  Field(r, 1) = Val_int(0);               /* cdr = the empty list [] */
  result = caml_alloc_small(2, 0);        /* Allocate the other cons cell */
  Field(result, 0) = Val_int(i1);         /* car = the integer i1 */
  Field(result, 1) = r;                   /* cdr = the first cons cell */
  CAMLreturn (result);
}
</pre><p>In the two examples above, the list is built bottom-up. Here is an
alternate way, that proceeds top-down. It is less efficient, but
illustrates the use of <span class="c003">caml_modify</span>.
</p><pre>value alloc_list_int(int i1, int i2)
{
  CAMLparam0 ();
  CAMLlocal2 (tail, r);

  r = caml_alloc_small(2, 0);             /* Allocate a cons cell */
  Field(r, 0) = Val_int(i1);              /* car = the integer i1 */
  Field(r, 1) = Val_int(0);               /* A dummy value
  tail = caml_alloc_small(2, 0);          /* Allocate the other cons cell */
  Field(tail, 0) = Val_int(i2);           /* car = the integer i2 */
  Field(tail, 1) = Val_int(0);            /* cdr = the empty list [] */
  caml_modify(&amp;Field(r, 1), tail);        /* cdr of the result = tail */
  CAMLreturn (r);
}
</pre><p>It would be incorrect to perform
<span class="c003">Field(r, 1) = tail</span> directly, because the allocation of <span class="c003">tail</span>
has taken place since <span class="c003">r</span> was allocated.</p>
<h2 class="section" id="sec447">20.6&#XA0;&#XA0;A complete example</h2>
<p>This section outlines how the functions from the Unix <span class="c003">curses</span> library
can be made available to OCaml programs. First of all, here is
the interface <span class="c003">curses.mli</span> that declares the <span class="c003">curses</span> primitives and
data types:
</p><pre>(* File curses.ml -- declaration of primitives and data types *)
type window                   (* The type "window" remains abstract *)
external initscr: unit -&gt; window = "caml_curses_initscr"
external endwin: unit -&gt; unit = "caml_curses_endwin"
external refresh: unit -&gt; unit = "caml_curses_refresh"
external wrefresh : window -&gt; unit = "caml_curses_wrefresh"
external newwin: int -&gt; int -&gt; int -&gt; int -&gt; window = "caml_curses_newwin"
external addch: char -&gt; unit = "caml_curses_addch"
external mvwaddch: window -&gt; int -&gt; int -&gt; char -&gt; unit = "caml_curses_mvwaddch"
external addstr: string -&gt; unit = "caml_curses_addstr"
external mvwaddstr: window -&gt; int -&gt; int -&gt; string -&gt; unit
         = "caml_curses_mvwaddstr"
(* lots more omitted *)
</pre><p>To compile this interface:
</p><pre>        ocamlc -c curses.ml
</pre><p>
To implement these functions, we just have to provide the stub code;
the core functions are already implemented in the <span class="c003">curses</span> library.
The stub code file, <span class="c003">curses_stubs.c</span>, looks like this:
</p><pre>/* File curses_stubs.c -- stub code for curses */
#include &lt;curses.h&gt;
#include &lt;caml/mlvalues.h&gt;
#include &lt;caml/memory.h&gt;
#include &lt;caml/alloc.h&gt;
#include &lt;caml/custom.h&gt;

/* Encapsulation of opaque window handles (of type WINDOW *)
   as OCaml custom blocks. */

static struct custom_operations curses_window_ops = {
  "fr.inria.caml.curses_windows",
  custom_finalize_default,
  custom_compare_default,
  custom_hash_default,
  custom_serialize_default,
  custom_deserialize_default,
  custom_compare_ext_default
};

/* Accessing the WINDOW * part of an OCaml custom block */
#define Window_val(v) (*((WINDOW **) Data_custom_val(v)))

/* Allocating an OCaml custom block to hold the given WINDOW * */
static value alloc_window(WINDOW * w)
{
  value v = alloc_custom(&amp;curses_window_ops, sizeof(WINDOW *), 0, 1);
  Window_val(v) = w;
  return v;
}

value caml_curses_initscr(value unit)
{
  CAMLparam1 (unit);
  CAMLreturn (alloc_window(initscr()));
}

value caml_curses_endwin(value unit)
{
  CAMLparam1 (unit);
  endwin();
  CAMLreturn (Val_unit);
}

value caml_curses_refresh(value unit)
{
  CAMLparam1 (unit);
  refresh();
  CAMLreturn (Val_unit);
}

value caml_curses_wrefresh(value win)
{
  CAMLparam1 (win);
  wrefresh(Window_val(win));
  CAMLreturn (Val_unit);
}

value caml_curses_newwin(value nlines, value ncols, value x0, value y0)
{
  CAMLparam4 (nlines, ncols, x0, y0);
  CAMLreturn (alloc_window(newwin(Int_val(nlines), Int_val(ncols),
                                  Int_val(x0), Int_val(y0))));
}

value caml_curses_addch(value c)
{
  CAMLparam1 (c);
  addch(Int_val(c));            /* Characters are encoded like integers */
  CAMLreturn (Val_unit);
}

value caml_curses_mvwaddch(value win, value x, value y, value c)
{
  CAMLparam4 (win, x, y, c);
  mvwaddch(Window_val(win), Int_val(x), Int_val(y), Int_val(c));
  CAMLreturn (Val_unit);
}

value caml_curses_addstr(value s)
{
  CAMLparam1 (s);
  addstr(String_val(s));
  CAMLreturn (Val_unit);
}

value caml_curses_mvwaddstr(value win, value x, value y, value s)
{
  CAMLparam4 (win, x, y, s);
  mvwaddstr(Window_val(win), Int_val(x), Int_val(y), String_val(s));
  CAMLreturn (Val_unit);
}

/* This goes on for pages. */
</pre><p>
The file <span class="c003">curses_stubs.c</span> can be compiled with:
</p><pre>        cc -c -I`ocamlc -where` curses_stubs.c
</pre><p>or, even simpler,
</p><pre>        ocamlc -c curses_stubs.c
</pre><p>(When passed a <span class="c003">.c</span> file, the <span class="c003">ocamlc</span> command simply calls the C
compiler on that file, with the right <span class="c003">-I</span> option.)</p><p>Now, here is a sample OCaml program <span class="c003">prog.ml</span> that uses the <span class="c003">curses</span>
module:
</p><pre>(* File prog.ml -- main program using curses *)
open Curses;;
let main_window = initscr () in
let small_window = newwin 10 5 20 10 in
  mvwaddstr main_window 10 2 "Hello";
  mvwaddstr small_window 4 3 "world";
  refresh();
  Unix.sleep 5;
  endwin()
</pre><p>To compile and link this program, run:
</p><pre>       ocamlc -custom -o prog unix.cma curses.cmo prog.ml curses_stubs.o -cclib -lcurses
</pre><p>(On some machines, you may need to put
<span class="c003">-cclib -lcurses -cclib -ltermcap</span> or <span class="c003">-cclib -ltermcap</span>
instead of <span class="c003">-cclib -lcurses</span>.)</p>
<h2 class="section" id="sec448">20.7&#XA0;&#XA0;Advanced topic: callbacks from C to OCaml</h2>
<p> <a id="s:callback"></a>
</p><p>So far, we have described how to call C functions from OCaml. In this
section, we show how C functions can call OCaml functions, either as
callbacks (OCaml calls C which calls OCaml), or with the main program
written in C.</p>
<h3 class="subsection" id="sec449">20.7.1&#XA0;&#XA0;Applying OCaml closures from C</h3>
<p> <a id="s:callbacks"></a></p><p>C functions can apply OCaml function values (closures) to OCaml values.
The following functions are provided to perform the applications:
</p><ul class="itemize"><li class="li-itemize">
<span class="c003">caml_callback(</span><span class="c009">f, a</span><span class="c003">)</span> applies the functional value <span class="c009">f</span> to
the value <span class="c009">a</span> and returns the value returned by&#XA0;<span class="c009">f</span>.
</li><li class="li-itemize"><span class="c003">caml_callback2(</span><span class="c009">f, a, b</span><span class="c003">)</span> applies the functional value <span class="c009">f</span>
(which is assumed to be a curried OCaml function with two arguments) to
<span class="c009">a</span> and <span class="c009">b</span>.
</li><li class="li-itemize"><span class="c003">caml_callback3(</span><span class="c009">f, a, b, c</span><span class="c003">)</span> applies the functional value <span class="c009">f</span>
(a curried OCaml function with three arguments) to <span class="c009">a</span>, <span class="c009">b</span> and <span class="c009">c</span>.
</li><li class="li-itemize"><span class="c003">caml_callbackN(</span><span class="c009">f, n, args</span><span class="c003">)</span> applies the functional value <span class="c009">f</span>
to the <span class="c009">n</span> arguments contained in the array of values <span class="c009">args</span>.
</li></ul><p>
If the function <span class="c009">f</span> does not return, but raises an exception that
escapes the scope of the application, then this exception is
propagated to the next enclosing OCaml code, skipping over the C
code. That is, if an OCaml function <span class="c009">f</span> calls a C function <span class="c009">g</span> that
calls back an OCaml function <span class="c009">h</span> that raises a stray exception, then the
execution of <span class="c009">g</span> is interrupted and the exception is propagated back
into <span class="c009">f</span>.</p><p>If the C code wishes to catch exceptions escaping the OCaml function,
it can use the functions <span class="c003">caml_callback_exn</span>, <span class="c003">caml_callback2_exn</span>,
<span class="c003">caml_callback3_exn</span>, <span class="c003">caml_callbackN_exn</span>. These functions take the same
arguments as their non-<span class="c003">_exn</span> counterparts, but catch escaping
exceptions and return them to the C code. The return value <span class="c009">v</span> of the
<span class="c003">caml_callback*_exn</span> functions must be tested with the macro
<span class="c003">Is_exception_result(</span><span class="c009">v</span><span class="c003">)</span>. If the macro returns &#X201C;false&#X201D;, no
exception occured, and <span class="c009">v</span> is the value returned by the OCaml
function. If <span class="c003">Is_exception_result(</span><span class="c009">v</span><span class="c003">)</span> returns &#X201C;true&#X201D;,
an exception escaped, and its value (the exception descriptor) can be
recovered using <span class="c003">Extract_exception(</span><span class="c009">v</span><span class="c003">)</span>.</p>
<h5 class="paragraph" id="sec450">Warning:</h5>
<p> If the OCaml function returned with an exception,
<span class="c003">Extract_exception</span> should be applied to the exception result prior
to calling a function that may trigger garbage collection.
Otherwise, if <span class="c009">v</span> is reachable during garbage collection, the runtime
can crash since <span class="c009">v</span> does not contain a valid value.</p><p>Example:
</p><pre>    value call_caml_f_ex(value closure, value arg)
    {
      CAMLparam2(closure, arg);
      CAMLlocal2(res, tmp);
      res = caml_callback_exn(closure, arg);
      if(Is_exception_result(res)) {
        res = Extract_exception(res);
        tmp = caml_alloc(3, 0); /* Safe to allocate: res contains valid value. */
        ...
      }
      CAMLreturn (res);
    }
</pre>
<h3 class="subsection" id="sec451">20.7.2&#XA0;&#XA0;Obtaining or registering OCaml closures for use in C functions</h3>
<p>There are two ways to obtain OCaml function values (closures) to
be passed to the <span class="c003">callback</span> functions described above. One way is to
pass the OCaml function as an argument to a primitive function. For
example, if the OCaml code contains the declaration
</p><pre>    external apply : ('a -&gt; 'b) -&gt; 'a -&gt; 'b = "caml_apply"
</pre><p>the corresponding C stub can be written as follows:
</p><pre>    CAMLprim value caml_apply(value vf, value vx)
    {
      CAMLparam2(vf, vx);
      CAMLlocal1(vy);
      vy = caml_callback(vf, vx);
      CAMLreturn(vy);
    }
</pre><p>
Another possibility is to use the registration mechanism provided by
OCaml. This registration mechanism enables OCaml code to register
OCaml functions under some global name, and C code to retrieve the
corresponding closure by this global name.</p><p>On the OCaml side, registration is performed by evaluating
<span class="c003">Callback.register</span> <span class="c009">n v</span>. Here, <span class="c009">n</span> is the global name
(an arbitrary string) and <span class="c009">v</span> the OCaml value. For instance:
</p><pre>    let f x = print_string "f is applied to "; print_int x; print_newline()
    let _ = Callback.register "test function" f
</pre><p>
On the C side, a pointer to the value registered under name <span class="c009">n</span> is
obtained by calling <span class="c003">caml_named_value(</span><span class="c009">n</span><span class="c003">)</span>. The returned
pointer must then be dereferenced to recover the actual OCaml value.
If no value is registered under the name <span class="c009">n</span>, the null pointer is
returned. For example, here is a C wrapper that calls the OCaml function <span class="c003">f</span>
above:
</p><pre>    void call_caml_f(int arg)
    {
        caml_callback(*caml_named_value("test function"), Val_int(arg));
    }
</pre><p>
The pointer returned by <span class="c003">caml_named_value</span> is constant and can safely
be cached in a C variable to avoid repeated name lookups. On the other
hand, the value pointed to can change during garbage collection and
must always be recomputed at the point of use. Here is a more
efficient variant of <span class="c003">call_caml_f</span> above that calls <span class="c003">caml_named_value</span>
only once:
</p><pre>    void call_caml_f(int arg)
    {
        static value * closure_f = NULL;
        if (closure_f == NULL) {
            /* First time around, look up by name */
            closure_f = caml_named_value("test function");
        }
        caml_callback(*closure_f, Val_int(arg));
    }
</pre>
<h3 class="subsection" id="sec452">20.7.3&#XA0;&#XA0;Registering OCaml exceptions for use in C functions</h3>
<p> <a id="s:register-exn"></a></p><p>The registration mechanism described above can also be used to
communicate exception identifiers from OCaml to C. The OCaml code
registers the exception by evaluating
<span class="c003">Callback.register_exception</span> <span class="c009">n exn</span>, where <span class="c009">n</span> is an
arbitrary name and <span class="c009">exn</span> is an exception value of the
exception to register. For example:
</p><pre>    exception Error of string
    let _ = Callback.register_exception "test exception" (Error "any string")
</pre><p>The C code can then recover the exception identifier using
<span class="c003">caml_named_value</span> and pass it as first argument to the functions
<span class="c003">raise_constant</span>, <span class="c003">raise_with_arg</span>, and <span class="c003">raise_with_string</span> (described
in section&#XA0;<a href="#s%3Ac-exceptions">20.4.5</a>) to actually raise the exception. For
example, here is a C function that raises the <span class="c003">Error</span> exception with
the given argument:
</p><pre>    void raise_error(char * msg)
    {
        caml_raise_with_string(*caml_named_value("test exception"), msg);
    }
</pre>
<h3 class="subsection" id="sec453">20.7.4&#XA0;&#XA0;Main program in C</h3>
<p> <a id="s:main-c"></a></p><p>In normal operation, a mixed OCaml/C program starts by executing the
OCaml initialization code, which then may proceed to call C
functions. We say that the main program is the OCaml code. In some
applications, it is desirable that the C code plays the role of the
main program, calling OCaml functions when needed. This can be achieved as
follows:
</p><ul class="itemize"><li class="li-itemize">
The C part of the program must provide a <span class="c003">main</span> function,
which will override the default <span class="c003">main</span> function provided by the OCaml
runtime system. Execution will start in the user-defined <span class="c003">main</span> function
just like for a regular C program.</li><li class="li-itemize">At some point, the C code must call <span class="c003">caml_main(argv)</span> to
initialize the OCaml code. The <span class="c003">argv</span> argument is a C array of strings
(type <span class="c003">char **</span>), terminated with a <span class="c003">NULL</span> pointer,
which represents the command-line arguments, as
passed as second argument to <span class="c003">main</span>. The OCaml array <span class="c003">Sys.argv</span> will
be initialized from this parameter. For the bytecode compiler,
<span class="c003">argv[0]</span> and <span class="c003">argv[1]</span> are also consulted to find the file containing
the bytecode.</li><li class="li-itemize">The call to <span class="c003">caml_main</span> initializes the OCaml runtime system,
loads the bytecode (in the case of the bytecode compiler), and
executes the initialization code of the OCaml program. Typically, this
initialization code registers callback functions using <span class="c003">Callback.register</span>.
Once the OCaml initialization code is complete, control returns to the
C code that called <span class="c003">caml_main</span>.</li><li class="li-itemize">The C code can then invoke OCaml functions using the callback
mechanism (see section&#XA0;<a href="#s%3Acallbacks">20.7.1</a>).
</li></ul>
<h3 class="subsection" id="sec454">20.7.5&#XA0;&#XA0;Embedding the OCaml code in the C code</h3>
<p> <a id="s:embedded-code"></a></p><p>The bytecode compiler in custom runtime mode (<span class="c003">ocamlc -custom</span>)
normally appends the bytecode to the executable file containing the
custom runtime. This has two consequences. First, the final linking
step must be performed by <span class="c003">ocamlc</span>. Second, the OCaml runtime library
must be able to find the name of the executable file from the
command-line arguments. When using <span class="c003">caml_main(argv)</span> as in
section&#XA0;<a href="#s%3Amain-c">20.7.4</a>, this means that <span class="c003">argv[0]</span> or <span class="c003">argv[1]</span> must
contain the executable file name.</p><p>An alternative is to embed the bytecode in the C code. The
<span class="c003">-output-obj</span> option to <span class="c003">ocamlc</span> is provided for this purpose. It
causes the <span class="c003">ocamlc</span> compiler to output a C object file (<span class="c003">.o</span> file,
<span class="c003">.obj</span> under Windows) containing the bytecode for the OCaml part of the
program, as well as a <span class="c003">caml_startup</span> function. The C object file
produced by <span class="c003">ocamlc -output-obj</span> can then be linked with C code using
the standard C compiler, or stored in a C library.</p><p>The <span class="c003">caml_startup</span> function must be called from the main C program in
order to initialize the OCaml runtime and execute the OCaml
initialization code. Just like <span class="c003">caml_main</span>, it takes one <span class="c003">argv</span>
parameter containing the command-line parameters. Unlike <span class="c003">caml_main</span>,
this <span class="c003">argv</span> parameter is used only to initialize <span class="c003">Sys.argv</span>, but not
for finding the name of the executable file.</p><p>The <span class="c003">caml_startup</span> function calls the uncaught exception handler (or
enters the debugger, if running under ocamldebug) if an exception escapes
from a top-level module initialiser. Such exceptions may be caught in the
C code by instead using the <span class="c003">caml_startup_exn</span> function and testing the result
using <span class="c003">Is_exception_result</span> (followed by <span class="c003">Extract_exception</span> if
appropriate).</p><p>The <span class="c003">-output-obj</span> option can also be used to obtain the C source file.
More interestingly, the same option can also produce directly a shared
library (<span class="c003">.so</span> file, <span class="c003">.dll</span> under Windows) that contains the OCaml
code, the OCaml runtime system and any other static C code given to
<span class="c003">ocamlc</span> (<span class="c003">.o</span>, <span class="c003">.a</span>, respectively, <span class="c003">.obj</span>, <span class="c003">.lib</span>). This use of
<span class="c003">-output-obj</span> is very similar to a normal linking step, but instead of
producing a main program that automatically runs the OCaml code, it
produces a shared library that can run the OCaml code on demand. The
three possible behaviors of <span class="c003">-output-obj</span> are selected according
to the extension of the resulting file (given with <span class="c003">-o</span>).</p><p>The native-code compiler <span class="c003">ocamlopt</span> also supports the <span class="c003">-output-obj</span>
option, causing it to output a C object file or a shared library
containing the native code for all OCaml modules on the command-line,
as well as the OCaml startup code. Initialization is performed by
calling <span class="c003">caml_startup</span> (or <span class="c003">caml_startup_exn</span>) as in the case of the
bytecode compiler.</p><p>For the final linking phase, in addition to the object file produced
by <span class="c003">-output-obj</span>, you will have to provide the OCaml runtime
library (<span class="c003">libcamlrun.a</span> for bytecode, <span class="c003">libasmrun.a</span> for native-code),
as well as all C libraries that are required by the OCaml libraries
used. For instance, assume the OCaml part of your program uses the
Unix library. With <span class="c003">ocamlc</span>, you should do:
</p><pre>
        ocamlc -output-obj -o camlcode.o unix.cma <span class="c009">other</span> .cmo <span class="c009">and</span> .cma <span class="c009">files</span>
        cc -o myprog <span class="c009">C objects and libraries</span> \
           camlcode.o -L&#X2018;ocamlc -where&#X2018; -lunix -lcamlrun
</pre><p>
With <span class="c003">ocamlopt</span>, you should do:
</p><pre>
        ocamlopt -output-obj -o camlcode.o unix.cmxa <span class="c009">other</span> .cmx <span class="c009">and</span> .cmxa <span class="c009">files</span>
        cc -o myprog <span class="c009">C objects and libraries</span> \
           camlcode.o -L&#X2018;ocamlc -where&#X2018; -lunix -lasmrun
</pre>
<h5 class="paragraph" id="sec455">Warning:</h5>
<p> On some ports, special options are required on the final
linking phase that links together the object file produced by the
<span class="c003">-output-obj</span> option and the remainder of the program. Those options
are shown in the configuration file <span class="c003">config/Makefile</span> generated during
compilation of OCaml, as the variable <span class="c003">LDFLAGS</span>.
</p><ul class="itemize"><li class="li-itemize">
Windows with the MSVC compiler: the object file produced by
OCaml have been compiled with the <span class="c003">/MD</span> flag, and therefore
all other object files linked with it should also be compiled with
<span class="c003">/MD</span>.
</li><li class="li-itemize">other systems: you may have to add one or more of <span class="c003">-lcurses</span>,
<span class="c003">-lm</span>, <span class="c003">-ldl</span>, depending on your OS and C compiler.
</li></ul>
<h5 class="paragraph" id="sec456">Stack backtraces.</h5>
<p> When OCaml bytecode produced by
<span class="c003">ocamlc -g</span> is embedded in a C program, no debugging information is
included, and therefore it is impossible to print stack backtraces on
uncaught exceptions. This is not the case when native code produced
by <span class="c003">ocamlopt -g</span> is embedded in a C program: stack backtrace
information is available, but the backtrace mechanism needs to be
turned on programmatically. This can be achieved from the OCaml side
by calling <span class="c003">Printexc.record_backtrace true</span> in the initialization of
one of the OCaml modules. This can also be achieved from the C side
by calling <span class="c003">caml_record_backtrace(Val_int(1));</span> in the OCaml-C glue code.</p>
<h5 class="paragraph" id="sec457">Unloading the runtime.</h5>
<p>In case the shared library produced with <span class="c003">-output-obj</span> is to be loaded and
unloaded repeatedly by a single process, care must be taken to unload the
OCaml runtime explicitly, in order to avoid various system resource leaks.</p><p>Since 4.05, <span class="c003">caml_shutdown</span> function can be used to shut the runtime down
gracefully, which equals the following:
</p><ul class="itemize"><li class="li-itemize">
Running the functions that were registered with <span class="c003">Pervasives.at_exit</span>.
</li><li class="li-itemize">Triggering finalization of allocated custom blocks (see
section&#XA0;<a href="#s%3Acustom">20.9</a>). For example, <span class="c003">Pervasives.in_channel</span> and
<span class="c003">Pervasives.out_channel</span> are represented by custom blocks that enclose file
descriptors, which are to be released.
</li><li class="li-itemize">Unloading the dependent shared libraries that were loaded by the runtime,
including <span class="c003">dynlink</span> plugins.
</li><li class="li-itemize">Freeing the memory blocks that were allocated by the runtime with
<span class="c003">malloc</span>. Inside C primitives, it is advised to use <span class="c003">caml_stat_*</span> functions
from <span class="c003">memory.h</span> for managing static (that is, non-moving) blocks of heap
memory, as all the blocks allocated with these functions are automatically
freed by <span class="c003">caml_shutdown</span>. For ensuring compatibility with legacy C stubs that
have used <span class="c003">caml_stat_*</span> incorrectly, this behaviour is only enabled if the
runtime is started with a specialized <span class="c003">caml_startup_pooled</span> function.
</li></ul><p>As a shared library may have several clients simultaneously, it is made for
convenience that <span class="c003">caml_startup</span> (and <span class="c003">caml_startup_pooled</span>) may be called
multiple times, given that each such call is paired with a corresponding call
to <span class="c003">caml_shutdown</span> (in a nested fashion). The runtime will be unloaded once
there are no outstanding calls to <span class="c003">caml_startup</span>.</p><p>Once a runtime is unloaded, it cannot be started up again without reloading the
shared library and reinitializing its static data. Therefore, at the moment, the
facility is only useful for building reloadable shared libraries.</p>
<h2 class="section" id="sec458">20.8&#XA0;&#XA0;Advanced example with callbacks</h2>
<p>This section illustrates the callback facilities described in
section&#XA0;<a href="#s%3Acallback">20.7</a>. We are going to package some OCaml functions
in such a way that they can be linked with C code and called from C
just like any C functions. The OCaml functions are defined in the
following <span class="c003">mod.ml</span> OCaml source:</p><pre>(* File mod.ml -- some "useful" OCaml functions *)

let rec fib n = if n &lt; 2 then 1 else fib(n-1) + fib(n-2)

let format_result n = Printf.sprintf "Result is: %d\n" n

(* Export those two functions to C *)

let _ = Callback.register "fib" fib
let _ = Callback.register "format_result" format_result
</pre><p>
Here is the C stub code for calling these functions from C:</p><pre>/* File modwrap.c -- wrappers around the OCaml functions */

#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;caml/mlvalues.h&gt;
#include &lt;caml/callback.h&gt;

int fib(int n)
{
  static value * fib_closure = NULL;
  if (fib_closure == NULL) fib_closure = caml_named_value("fib");
  return Int_val(caml_callback(*fib_closure, Val_int(n)));
}

char * format_result(int n)
{
  static value * format_result_closure = NULL;
  if (format_result_closure == NULL)
    format_result_closure = caml_named_value("format_result");
  return strdup(String_val(caml_callback(*format_result_closure, Val_int(n))));
  /* We copy the C string returned by String_val to the C heap
     so that it remains valid after garbage collection. */
}
</pre><p>
We now compile the OCaml code to a C object file and put it in a C
library along with the stub code in <span class="c003">modwrap.c</span> and the OCaml runtime system:
</p><pre>        ocamlc -custom -output-obj -o modcaml.o mod.ml
        ocamlc -c modwrap.c
        cp `ocamlc -where`/libcamlrun.a mod.a &amp;&amp; chmod +w mod.a
        ar r mod.a modcaml.o modwrap.o
</pre><p>(One can also use <span class="c003">ocamlopt -output-obj</span> instead of <span class="c003">ocamlc -custom -output-obj</span>. In this case, replace <span class="c003">libcamlrun.a</span> (the bytecode
runtime library) by <span class="c003">libasmrun.a</span> (the native-code runtime library).)</p><p>Now, we can use the two functions <span class="c003">fib</span> and <span class="c003">format_result</span> in any C
program, just like regular C functions. Just remember to call
<span class="c003">caml_startup</span> (or <span class="c003">caml_startup_exn</span>) once before.</p><pre>/* File main.c -- a sample client for the OCaml functions */

#include &lt;stdio.h&gt;
#include &lt;caml/callback.h&gt;

extern int fib(int n);
extern char * format_result(int n);

int main(int argc, char ** argv)
{
  int result;

  /* Initialize OCaml code */
  caml_startup(argv);
  /* Do some computation */
  result = fib(10);
  printf("fib(10) = %s\n", format_result(result));
  return 0;
}
</pre><p>
To build the whole program, just invoke the C compiler as follows:
</p><pre>        cc -o prog -I `ocamlc -where` main.c mod.a -lcurses
</pre><p>(On some machines, you may need to put <span class="c003">-ltermcap</span> or
<span class="c003">-lcurses -ltermcap</span> instead of <span class="c003">-lcurses</span>.)</p>
<h2 class="section" id="sec459">20.9&#XA0;&#XA0;Advanced topic: custom blocks</h2>
<p> <a id="s:custom"></a>
</p><p>Blocks with tag <span class="c003">Custom_tag</span> contain both arbitrary user data and a
pointer to a C struct, with type <span class="c003">struct custom_operations</span>, that
associates user-provided finalization, comparison, hashing,
serialization and deserialization functions to this block.</p>
<h3 class="subsection" id="sec460">20.9.1&#XA0;&#XA0;The <span class="c003">struct custom_operations</span></h3>
<p>The <span class="c003">struct custom_operations</span> is defined in <span class="c003">&lt;caml/custom.h&gt;</span> and
contains the following fields:
</p><ul class="itemize"><li class="li-itemize">
<span class="c003">char *identifier</span> <br>
A zero-terminated character string serving as an identifier for
serialization and deserialization operations.</li><li class="li-itemize"><span class="c003">void  (*finalize)(value v)</span> <br>
The <span class="c003">finalize</span> field contains a pointer to a C function that is called
when the block becomes unreachable and is about to be reclaimed.
The block is passed as first argument to the function.
The <span class="c003">finalize</span> field can also be <span class="c003">custom_finalize_default</span> to indicate that no
finalization function is associated with the block.</li><li class="li-itemize"><span class="c003">int (*compare)(value v1, value v2)</span> <br>
The <span class="c003">compare</span> field contains a pointer to a C function that is
called whenever two custom blocks are compared using OCaml&#X2019;s generic
comparison operators (<span class="c003">=</span>, <span class="c003">&lt;&gt;</span>, <span class="c003">&lt;=</span>, <span class="c003">&gt;=</span>, <span class="c003">&lt;</span>, <span class="c003">&gt;</span> and
<span class="c003">compare</span>). The C function should return 0 if the data contained in
the two blocks are structurally equal, a negative integer if the data
from the first block is less than the data from the second block, and
a positive integer if the data from the first block is greater than
the data from the second block.<p>The <span class="c003">compare</span> field can be set to <span class="c003">custom_compare_default</span>; this
default comparison function simply raises <span class="c003">Failure</span>.</p></li><li class="li-itemize"><span class="c003">int (*compare_ext)(value v1, value v2)</span> <br>
(Since 3.12.1)
The <span class="c003">compare_ext</span> field contains a pointer to a C function that is
called whenever one custom block and one unboxed integer are compared using OCaml&#X2019;s generic
comparison operators (<span class="c003">=</span>, <span class="c003">&lt;&gt;</span>, <span class="c003">&lt;=</span>, <span class="c003">&gt;=</span>, <span class="c003">&lt;</span>, <span class="c003">&gt;</span> and
<span class="c003">compare</span>). As in the case of the <span class="c003">compare</span> field, the C function
should return 0 if the two arguments are structurally equal, a
negative integer if the first argument compares less than the second
argument, and a positive integer if the first argument compares
greater than the second argument.<p>The <span class="c003">compare_ext</span> field can be set to <span class="c003">custom_compare_ext_default</span>; this
default comparison function simply raises <span class="c003">Failure</span>.</p></li><li class="li-itemize"><span class="c003">intnat (*hash)(value v)</span> <br>
The <span class="c003">hash</span> field contains a pointer to a C function that is called
whenever OCaml&#X2019;s generic hash operator (see module <span class="c003">Hashtbl</span>) is
applied to a custom block. The C function can return an arbitrary
integer representing the hash value of the data contained in the
given custom block. The hash value must be compatible with the
<span class="c003">compare</span> function, in the sense that two structurally equal data
(that is, two custom blocks for which <span class="c003">compare</span> returns 0) must have
the same hash value.<p>The <span class="c003">hash</span> field can be set to <span class="c003">custom_hash_default</span>, in which case
the custom block is ignored during hash computation.</p></li><li class="li-itemize"><span class="c003">void (*serialize)(value v, uintnat * wsize_32, uintnat * wsize_64)</span> <br>
The <span class="c003">serialize</span> field contains a pointer to a C function that is
called whenever the custom block needs to be serialized (marshaled)
using the OCaml functions <span class="c003">output_value</span> or <span class="c003">Marshal.to_...</span>.
For a custom block, those functions first write the identifier of the
block (as given by the <span class="c003">identifier</span> field) to the output stream,
then call the user-provided <span class="c003">serialize</span> function. That function is
responsible for writing the data contained in the custom block, using
the <span class="c003">serialize_...</span> functions defined in <span class="c003">&lt;caml/intext.h&gt;</span> and listed
below. The user-provided <span class="c003">serialize</span> function must then store in its
<span class="c003">wsize_32</span> and <span class="c003">wsize_64</span> parameters the sizes in bytes of the data
part of the custom block on a 32-bit architecture and on a 64-bit
architecture, respectively.<p>The <span class="c003">serialize</span> field can be set to <span class="c003">custom_serialize_default</span>,
in which case the <span class="c003">Failure</span> exception is raised when attempting to
serialize the custom block.</p></li><li class="li-itemize"><span class="c003">uintnat (*deserialize)(void * dst)</span> <br>
The <span class="c003">deserialize</span> field contains a pointer to a C function that is
called whenever a custom block with identifier <span class="c003">identifier</span> needs to
be deserialized (un-marshaled) using the OCaml functions <span class="c003">input_value</span>
or <span class="c003">Marshal.from_...</span>. This user-provided function is responsible for
reading back the data written by the <span class="c003">serialize</span> operation, using the
<span class="c003">deserialize_...</span> functions defined in <span class="c003">&lt;caml/intext.h&gt;</span> and listed
below. It must then rebuild the data part of the custom block
and store it at the pointer given as the <span class="c003">dst</span> argument. Finally, it
returns the size in bytes of the data part of the custom block.
This size must be identical to the <span class="c003">wsize_32</span> result of
the <span class="c003">serialize</span> operation if the architecture is 32 bits, or
<span class="c003">wsize_64</span> if the architecture is 64 bits.<p>The <span class="c003">deserialize</span> field can be set to <span class="c003">custom_deserialize_default</span>
to indicate that deserialization is not supported. In this case,
do not register the <span class="c003">struct custom_operations</span> with the deserializer
using <span class="c003">register_custom_operations</span> (see below).
</p></li></ul><p>Note: the <span class="c003">finalize</span>, <span class="c003">compare</span>, <span class="c003">hash</span>, <span class="c003">serialize</span> and <span class="c003">deserialize</span>
functions attached to custom block descriptors must never trigger a
garbage collection. Within these functions, do not call any of the
OCaml allocation functions, and do not perform a callback into OCaml
code. Do not use <span class="c003">CAMLparam</span> to register the parameters to these
functions, and do not use <span class="c003">CAMLreturn</span> to return the result.</p>
<h3 class="subsection" id="sec461">20.9.2&#XA0;&#XA0;Allocating custom blocks</h3>
<p>Custom blocks must be allocated via the <span class="c003">caml_alloc_custom</span> function:
</p><div class="center">
<span class="c003">caml_alloc_custom(</span><span class="c009">ops</span><span class="c003">, </span><span class="c009">size</span><span class="c003">, </span><span class="c009">used</span><span class="c003">, </span><span class="c009">max</span><span class="c003">)</span>
</div><p>
returns a fresh custom block, with room for <span class="c009">size</span> bytes of user
data, and whose associated operations are given by <span class="c009">ops</span> (a
pointer to a <span class="c003">struct custom_operations</span>, usually statically allocated
as a C global variable).</p><p>The two parameters <span class="c009">used</span> and <span class="c009">max</span> are used to control the
speed of garbage collection when the finalized object contains
pointers to out-of-heap resources. Generally speaking, the
OCaml incremental major collector adjusts its speed relative to the
allocation rate of the program. The faster the program allocates, the
harder the GC works in order to reclaim quickly unreachable blocks
and avoid having large amount of &#X201C;floating garbage&#X201D; (unreferenced
objects that the GC has not yet collected).</p><p>Normally, the allocation rate is measured by counting the in-heap size
of allocated blocks. However, it often happens that finalized
objects contain pointers to out-of-heap memory blocks and other resources
(such as file descriptors, X Windows bitmaps, etc.). For those
blocks, the in-heap size of blocks is not a good measure of the
quantity of resources allocated by the program.</p><p>The two arguments <span class="c009">used</span> and <span class="c009">max</span> give the GC an idea of how
much out-of-heap resources are consumed by the finalized block
being allocated: you give the amount of resources allocated to this
object as parameter <span class="c009">used</span>, and the maximum amount that you want
to see in floating garbage as parameter <span class="c009">max</span>. The units are
arbitrary: the GC cares only about the ratio <span class="c009">used</span> / <span class="c009">max</span>.</p><p>For instance, if you are allocating a finalized block holding an X
Windows bitmap of <span class="c009">w</span> by <span class="c009">h</span> pixels, and you&#X2019;d rather not
have more than 1 mega-pixels of unreclaimed bitmaps, specify
<span class="c009">used</span> = <span class="c009">w</span> * <span class="c009">h</span> and <span class="c009">max</span> = 1000000.</p><p>Another way to describe the effect of the <span class="c009">used</span> and <span class="c009">max</span>
parameters is in terms of full GC cycles. If you allocate many custom
blocks with <span class="c009">used</span> / <span class="c009">max</span> = 1 / <span class="c009">N</span>, the GC will then do one
full cycle (examining every object in the heap and calling
finalization functions on those that are unreachable) every <span class="c009">N</span>
allocations. For instance, if <span class="c009">used</span> = 1 and <span class="c009">max</span> = 1000,
the GC will do one full cycle at least every 1000 allocations of
custom blocks.</p><p>If your finalized blocks contain no pointers to out-of-heap resources,
or if the previous discussion made little sense to you, just take
<span class="c009">used</span> = 0 and <span class="c009">max</span> = 1. But if you later find that the
finalization functions are not called &#X201C;often enough&#X201D;, consider
increasing the <span class="c009">used</span> / <span class="c009">max</span> ratio.</p>
<h3 class="subsection" id="sec462">20.9.3&#XA0;&#XA0;Accessing custom blocks</h3>
<p>The data part of a custom block <span class="c009">v</span> can be
accessed via the pointer <span class="c003">Data_custom_val(</span><span class="c009">v</span><span class="c003">)</span>. This pointer
has type <span class="c003">void *</span> and should be cast to the actual type of the data
stored in the custom block.</p><p>The contents of custom blocks are not scanned by the garbage
collector, and must therefore not contain any pointer inside the OCaml
heap. In other terms, never store an OCaml <span class="c003">value</span> in a custom block,
and do not use <span class="c003">Field</span>, <span class="c003">Store_field</span> nor <span class="c003">caml_modify</span> to access the data
part of a custom block. Conversely, any C data structure (not
containing heap pointers) can be stored in a custom block.</p>
<h3 class="subsection" id="sec463">20.9.4&#XA0;&#XA0;Writing custom serialization and deserialization functions</h3>
<p>The following functions, defined in <span class="c003">&lt;caml/intext.h&gt;</span>, are provided to
write and read back the contents of custom blocks in a portable way.
Those functions handle endianness conversions when e.g. data is
written on a little-endian machine and read back on a big-endian machine.</p><div class="center"><table class="c000 cellpadding1" border=1><tr><td class="c014"><span class="c013">Function</span></td><td class="c014"><span class="c013">Action</span> </td></tr>
<tr><td class="c022">
<span class="c003">caml_serialize_int_1</span></td><td class="c021">Write a 1-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_serialize_int_2</span></td><td class="c021">Write a 2-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_serialize_int_4</span></td><td class="c021">Write a 4-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_serialize_int_8</span></td><td class="c021">Write a 8-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_serialize_float_4</span></td><td class="c021">Write a 4-byte float </td></tr>
<tr><td class="c022"><span class="c003">caml_serialize_float_8</span></td><td class="c021">Write a 8-byte float </td></tr>
<tr><td class="c022"><span class="c003">caml_serialize_block_1</span></td><td class="c021">Write an array of 1-byte quantities </td></tr>
<tr><td class="c022"><span class="c003">caml_serialize_block_2</span></td><td class="c021">Write an array of 2-byte quantities </td></tr>
<tr><td class="c022"><span class="c003">caml_serialize_block_4</span></td><td class="c021">Write an array of 4-byte quantities </td></tr>
<tr><td class="c022"><span class="c003">caml_serialize_block_8</span></td><td class="c021">Write an array of 8-byte quantities </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_uint_1</span></td><td class="c021">Read an unsigned 1-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_sint_1</span></td><td class="c021">Read a signed 1-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_uint_2</span></td><td class="c021">Read an unsigned 2-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_sint_2</span></td><td class="c021">Read a signed 2-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_uint_4</span></td><td class="c021">Read an unsigned 4-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_sint_4</span></td><td class="c021">Read a signed 4-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_uint_8</span></td><td class="c021">Read an unsigned 8-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_sint_8</span></td><td class="c021">Read a signed 8-byte integer </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_float_4</span></td><td class="c021">Read a 4-byte float </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_float_8</span></td><td class="c021">Read an 8-byte float </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_block_1</span></td><td class="c021">Read an array of 1-byte quantities </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_block_2</span></td><td class="c021">Read an array of 2-byte quantities </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_block_4</span></td><td class="c021">Read an array of 4-byte quantities </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_block_8</span></td><td class="c021">Read an array of 8-byte quantities </td></tr>
<tr><td class="c022"><span class="c003">caml_deserialize_error</span></td><td class="c021">Signal an error during deserialization;
<span class="c003">input_value</span> or <span class="c003">Marshal.from_...</span> raise a <span class="c003">Failure</span> exception after
cleaning up their internal data structures </td></tr>
</table></div><p>Serialization functions are attached to the custom blocks to which
they apply. Obviously, deserialization functions cannot be attached
this way, since the custom block does not exist yet when
deserialization begins! Thus, the <span class="c003">struct custom_operations</span> that
contain deserialization functions must be registered with the
deserializer in advance, using the <span class="c003">register_custom_operations</span>
function declared in <span class="c003">&lt;caml/custom.h&gt;</span>. Deserialization proceeds by
reading the identifier off the input stream, allocating a custom block
of the size specified in the input stream, searching the registered
<span class="c003">struct custom_operation</span> blocks for one with the same identifier, and
calling its <span class="c003">deserialize</span> function to fill the data part of the custom block.</p>
<h3 class="subsection" id="sec464">20.9.5&#XA0;&#XA0;Choosing identifiers</h3>
<p>Identifiers in <span class="c003">struct custom_operations</span> must be chosen carefully,
since they must identify uniquely the data structure for serialization
and deserialization operations. In particular, consider including a
version number in the identifier; this way, the format of the data can
be changed later, yet backward-compatible deserialisation functions
can be provided.</p><p>Identifiers starting with <span class="c003">_</span> (an underscore character) are reserved
for the OCaml runtime system; do not use them for your custom
data. We recommend to use a URL
(<span class="c003">http://mymachine.mydomain.com/mylibrary/version-number</span>)
or a Java-style package name
(<span class="c003">com.mydomain.mymachine.mylibrary.version-number</span>)
as identifiers, to minimize the risk of identifier collision.</p>
<h3 class="subsection" id="sec465">20.9.6&#XA0;&#XA0;Finalized blocks</h3>
<p>Custom blocks generalize the finalized blocks that were present in
OCaml prior to version 3.00. For backward compatibility, the
format of custom blocks is compatible with that of finalized blocks,
and the <span class="c003">alloc_final</span> function is still available to allocate a custom
block with a given finalization function, but default comparison,
hashing and serialization functions. <span class="c003">caml_alloc_final(</span><span class="c009">n</span><span class="c003">, </span><span class="c009">f</span><span class="c003">, </span><span class="c009">used</span><span class="c003">, </span><span class="c009">max</span><span class="c003">)</span> returns a fresh custom block of
size <span class="c009">n</span> words, with finalization function <span class="c009">f</span>. The first
word is reserved for storing the custom operations; the other
<span class="c009">n</span>-1 words are available for your data. The two parameters
<span class="c009">used</span> and <span class="c009">max</span> are used to control the speed of garbage
collection, as described for <span class="c003">caml_alloc_custom</span>.</p>
<h2 class="section" id="sec466">20.10&#XA0;&#XA0;Advanced topic: cheaper C call</h2>
<p>
<a id="s:C-cheaper-call"></a></p><p>This section describe how to make calling C functions cheaper.</p><p><span class="c013">Note:</span> this only applies to the native compiler. So whenever you
use any of these methods, you have to provide an alternative byte-code
stub that ignores all the special annotations.</p>
<h3 class="subsection" id="sec467">20.10.1&#XA0;&#XA0;Passing unboxed values</h3>
<p>We said earlier that all OCaml objects are represented by the C type
<span class="c003">value</span>, and one has to use macros such as <span class="c003">Int_val</span> to decode data from
the <span class="c003">value</span> type. It is however possible to tell the OCaml native-code
compiler to do this for us and pass arguments unboxed to the C function.
Similarly it is possible to tell OCaml to expect the result unboxed and box
it for us.</p><p>The motivation is that, by letting &#X2018;ocamlopt&#X2018; deal with boxing, it can
often decide to suppress it entirely.</p><p>For instance let&#X2019;s consider this example:</p><pre>external foo : float -&gt; float -&gt; float = "foo"

let f a b =
  let len = Array.length a in
  assert (Array.length b = len);
  let res = Array.make len 0. in
  for i = 0 to len - 1 do
    res.(i) &lt;- foo a.(i) b.(i)
  done
</pre><p>
Float arrays are unboxed in OCaml, however the C function <span class="c003">foo</span> expect
its arguments as boxed floats and returns a boxed float. Hence the
OCaml compiler has no choice but to box <span class="c003">a.(i)</span> and <span class="c003">b.(i)</span> and unbox
the result of <span class="c003">foo</span>. This results in the allocation of <span class="c003">3 * len</span>
temporary float values.</p><p>Now if we annotate the arguments and result with <span class="c003">[@unboxed]</span>, the
native-code compiler will be able to avoid all these allocations:</p><pre>external foo
  :  (float [@unboxed])
  -&gt; (float [@unboxed])
  -&gt; (float [@unboxed])
  = "foo_byte" "foo"
</pre><p>
In this case the C functions must look like:</p><pre>CAMLprim double foo(double a, double b)
{
  ...
}

CAMLprim value foo_byte(value a, value b)
{
  return caml_copy_double(foo(Double_val(a), Double_val(b)))
}
</pre><p>
For convenicence, when all arguments and the result are annotated with
<span class="c003">[@unboxed]</span>, it is possible to put the attribute only once on the
declaration itself. So we can also write instead:</p><pre>external foo : float -&gt; float -&gt; float = "foo_byte" "foo" [@@unboxed]
</pre><p>
The following table summarize what OCaml types can be unboxed, and
what C types should be used in correspondence:</p><div class="center"><table class="c000 cellpadding1" border=1><tr><td class="c014"><span class="c013">OCaml type</span></td><td class="c014"><span class="c013">C type</span> </td></tr>
<tr><td class="c016">
<span class="c003">float</span></td><td class="c016"><span class="c003">double</span> </td></tr>
<tr><td class="c016"><span class="c003">int32</span></td><td class="c016"><span class="c003">int32_t</span> </td></tr>
<tr><td class="c016"><span class="c003">int64</span></td><td class="c016"><span class="c003">int64_t</span> </td></tr>
<tr><td class="c016"><span class="c003">nativeint</span></td><td class="c016"><span class="c003">intnat</span> </td></tr>
</table></div><p>Similarly, it is possible to pass untagged OCaml integers between
OCaml and C. This is done by annotating the arguments and/or result
with <span class="c003">[@untagged]</span>:</p><pre>external f : string -&gt; (int [@untagged]) = "f_byte" "f"
</pre><p>
The corresponding C type must be <span class="c003">intnat</span>.</p><p><span class="c013">Note:</span> do not use the C <span class="c003">int</span> type in correspondence with <span class="c003">(int [@untagged])</span>. This is because they often differ in size.</p>
<h3 class="subsection" id="sec468">20.10.2&#XA0;&#XA0;Direct C call</h3>
<p>In order to be able to run the garbage collector in the middle of
a C function, the OCaml native-code compiler generates some bookkeeping
code around C calls. Technically it wraps every C call with the C function
<span class="c003">caml_c_call</span> which is part of the OCaml runtime.</p><p>For small functions that are called repeatedly, this indirection can have
a big impact on performances. However this is not needed if we know that
the C function doesn&#X2019;t allocate and doesn&#X2019;t raise exceptions. We can
instruct the OCaml native-code compiler of this fact by annotating the
external declaration with the attribute <span class="c003">[@@noalloc]</span>:</p><pre>external bar : int -&gt; int -&gt; int = "foo" [@@noalloc]
</pre><p>
In this case calling <span class="c003">bar</span> from OCaml is as cheap as calling any other
OCaml function, except for the fact that the OCaml compiler can&#X2019;t
inline C functions...</p>
<h3 class="subsection" id="sec469">20.10.3&#XA0;&#XA0;Example: calling C library functions without indirection</h3>
<p>Using these attributes, it is possible to call C library functions
with no indirection. For instance many math functions are defined this
way in the OCaml standard library:</p><pre>external sqrt : float -&gt; float = "caml_sqrt_float" "sqrt"
  [@@unboxed] [@@noalloc]
(** Square root. *)

external exp : float -&gt; float = "caml_exp_float" "exp" [@@unboxed] [@@noalloc]
(** Exponential. *)

external log : float -&gt; float = "caml_log_float" "log" [@@unboxed] [@@noalloc]
(** Natural logarithm. *)
</pre>
<h2 class="section" id="sec470">20.11&#XA0;&#XA0;Advanced topic: multithreading</h2>
<p>
<a id="s:C-multithreading"></a></p><p>Using multiple threads (shared-memory concurrency) in a mixed OCaml/C
application requires special precautions, which are described in this
section.</p>
<h3 class="subsection" id="sec471">20.11.1&#XA0;&#XA0;Registering threads created from C</h3>
<p>Callbacks from C to OCaml are possible only if the calling thread is
known to the OCaml run-time system. Threads created from OCaml (through
the <span class="c003">Thread.create</span> function of the system threads library) are
automatically known to the run-time system. If the application
creates additional threads from C and wishes to callback into OCaml
code from these threads, it must first register them with the run-time
system. The following functions are declared in the include file
<span class="c003">&lt;caml/threads.h&gt;</span>.</p><ul class="itemize"><li class="li-itemize">
<span class="c003">caml_c_thread_register()</span> registers the calling thread with the OCaml
run-time system. Returns 1 on success, 0 on error. Registering an
already-register thread does nothing and returns 0.
</li><li class="li-itemize"><span class="c003">caml_c_thread_unregister()</span> must be called before the thread
terminates, to unregister it from the OCaml run-time system.
Returns 1 on success, 0 on error. If the calling thread was not
previously registered, does nothing and returns 0.
</li></ul>
<h3 class="subsection" id="sec472">20.11.2&#XA0;&#XA0;Parallel execution of long-running C code</h3>
<p>The OCaml run-time system is not reentrant: at any time, at most one
thread can be executing OCaml code or C code that uses the OCaml
run-time system. Technically, this is enforced by a &#X201C;master lock&#X201D;
that any thread must hold while executing such code.</p><p>When OCaml calls the C code implementing a primitive, the master lock
is held, therefore the C code has full access to the facilities of the
run-time system. However, no other thread can execute OCaml code
concurrently with the C code of the primitive.</p><p>If a C primitive runs for a long time or performs potentially blocking
input-output operations, it can explicitly release the master lock,
enabling other OCaml threads to run concurrently with its operations.
The C code must re-acquire the master lock before returning to OCaml.
This is achieved with the following functions, declared in
the include file <span class="c003">&lt;caml/threads.h&gt;</span>.</p><ul class="itemize"><li class="li-itemize">
<span class="c003">caml_release_runtime_system()</span>
The calling thread releases the master lock and other OCaml resources,
enabling other threads to run OCaml code in parallel with the execution
of the calling thread.
</li><li class="li-itemize"><span class="c003">caml_acquire_runtime_system()</span>
The calling thread re-acquires the master lock and other OCaml
resources. It may block until no other thread uses the OCaml run-time
system.
</li></ul><p>After <span class="c003">caml_release_runtime_system()</span> was called and until
<span class="c003">caml_acquire_runtime_system()</span> is called, the C code must not access
any OCaml data, nor call any function of the run-time system, nor call
back into OCaml code. Consequently, arguments provided by OCaml to the
C primitive must be copied into C data structures before calling
<span class="c003">caml_release_runtime_system()</span>, and results to be returned to OCaml
must be encoded as OCaml values after <span class="c003">caml_acquire_runtime_system()</span>
returns.</p><p>Example: the following C primitive invokes <span class="c003">gethostbyname</span> to find the
IP address of a host name. The <span class="c003">gethostbyname</span> function can block for
a long time, so we choose to release the OCaml run-time system while it
is running.
</p><pre>CAMLprim stub_gethostbyname(value vname)
{
  CAMLparam1 (vname);
  CAMLlocal1 (vres);
  struct hostent * h;
  char * name;

  /* Copy the string argument to a C string, allocated outside the
     OCaml heap. */
  name = caml_stat_strdup(String_val(vname));
  /* Release the OCaml run-time system */
  caml_release_runtime_system();
  /* Resolve the name */
  h = gethostbyname(name);
  /* Free the copy of the string, which we might as well do before
     acquiring the runtime system to benefit from parallelism. */
  caml_stat_free(name);
  /* Re-acquire the OCaml run-time system */
  caml_acquire_runtime_system();
  /* Encode the relevant fields of h as the OCaml value vres */
  ... /* Omitted */
  /* Return to OCaml */
  CAMLreturn (vres);
}
</pre><p>
Callbacks from C to OCaml must be performed while holding the master
lock to the OCaml run-time system. This is naturally the case if the
callback is performed by a C primitive that did not release the
run-time system. If the C primitive released the run-time system
previously, or the callback is performed from other C code that was
not invoked from OCaml (e.g. an event loop in a GUI application), the
run-time system must be acquired before the callback and released
after:
</p><pre>  caml_acquire_runtime_system();
  /* Resolve OCaml function vfun to be invoked */
  /* Build OCaml argument varg to the callback */
  vres = callback(vfun, varg);
  /* Copy relevant parts of result vres to C data structures */
  caml_release_runtime_system();
</pre><p>
Note: the <span class="c003">acquire</span> and <span class="c003">release</span> functions described above were
introduced in OCaml 3.12. Older code uses the following historical
names, declared in <span class="c003">&lt;caml/signals.h&gt;</span>:
</p><ul class="itemize"><li class="li-itemize">
<span class="c003">caml_enter_blocking_section</span> as an alias for
<span class="c003">caml_release_runtime_system</span>
</li><li class="li-itemize"><span class="c003">caml_leave_blocking_section</span> as an alias for
<span class="c003">caml_acquire_runtime_system</span>
</li></ul><p>
Intuition: a &#X201C;blocking section&#X201D; is a piece of C code that does not
use the OCaml run-time system, typically a blocking input/output operation.</p>
<h2 class="section" id="sec473">20.12&#XA0;&#XA0;Advanced topic: interfacing with Windows Unicode APIs</h2>
<p>
<a id="s:interfacing-windows-unicode-apis"></a></p><p>This section contains some general guidelines for writing C stubs that use
Windows Unicode APIs.</p><p><span class="c013">Note:</span> This is an experimental feature of OCaml: the set of APIs below, as
well as their exact semantics are not final and subject to change in future
releases.</p><p>The OCaml system under Windows can be configured at build time in one of two
modes:</p><ul class="itemize"><li class="li-itemize"><span class="c013">legacy mode:</span> All path names, environment variables, command line
arguments, etc. on the OCaml side are assumed to be encoded using the current
8-bit code page of the system.</li><li class="li-itemize"><span class="c013">Unicode mode:</span> All path names, environment variables, command line
arguments, etc. on the OCaml side are assumed to be encoded using UTF-8.</li></ul><p>In what follows, we say that a string has the <em>OCaml encoding</em> if it is
encoded in UTF-8 when in Unicode mode, in the current code page in legacy mode,
or is an arbitrary string under Unix. A string has the <em>platform encoding</em>
if it is encoded in UTF-16 under Windows or is an arbitrary string under Unix.</p><p>From the point of view of the writer of C stubs, the challenges of interacting
with Windows Unicode APIs are twofold:</p><ul class="itemize"><li class="li-itemize">The Windows API uses the UTF-16 encoding to support Unicode. The runtime
system performs the necessary conversions so that the OCaml programmer only
needs to deal with the OCaml encoding. C stubs that call Windows Unicode APIs
need to use specific runtime functions to perform the necessary conversions in a
compatible way.</li><li class="li-itemize">When writing stubs that need to be compiled under both Windows and Unix,
the stubs need to be written in a way that allow the necessary conversions under
Windows but that also work under Unix, where typically nothing particular needs
to be done to support Unicode.</li></ul><p>The native C character type under Windows is <span class="c003">WCHAR</span>, two bytes wide, while
under Unix it is <span class="c003">char</span>, one byte wide. A type <span class="c003">char_os</span> is defined in
<span class="c003">&lt;caml/misc.h&gt;</span> that stands for the concrete C character type of each
platform. Strings in the platform encoding are of type <span class="c003">char_os *</span>.</p><p>The following functions are exposed to help write compatible C stubs. To use
them, you need to include both <span class="c003">&lt;caml/misc.h&gt;</span> and <span class="c003">&lt;caml/osdeps.h&gt;</span>.</p><ul class="itemize"><li class="li-itemize"><span class="c003">char_os* caml_stat_strdup_to_os(const char *)</span> copies the argument while
translating from OCaml encoding to the platform encoding. This function is
typically used to convert the <span class="c003">char *</span> underlying an OCaml string before passing
it to an operating system API that takes an Unicode argument. Under Unix, it is
equivalent to <span class="c003">caml_stat_strdup</span>.<p><span class="c013">Note:</span> For maximum backwards compatibility in Unicode mode, if the argument
is not a valid UTF-8 string, this function will fall back to assuming that it is
encoded in the current code page.</p></li><li class="li-itemize"><span class="c003">char* caml_stat_strdup_of_os(const char_os *)</span> copies the argument while
translating from the platform encoding to the OCaml encoding. It is the inverse
of <span class="c003">caml_stat_strdup_to_os</span>. This function is typically used to convert a string
obtained from the operating system before passing it on to OCaml code. Under
Unix, it is equivalent to <span class="c003">caml_stat_strdup</span>.</li><li class="li-itemize"><span class="c003">value caml_copy_string_of_os(char_os *)</span> allocates an OCaml string with
contents equal to the argument string converted to the OCaml encoding. This
function is essentially equivalent to <span class="c003">caml_stat_strdup_of_os</span> followed by
<span class="c003">caml_copy_string</span>, except that it avoids the allocation of the intermediate
string returned by <span class="c003">caml_stat_strdup_of_os</span>. Under Unix, it is equivalent to
<span class="c003">caml_copy_string</span>.</li></ul><p><span class="c013">Note:</span> The strings returned by <span class="c003">caml_stat_strdup_to_os</span> and
<span class="c003">caml_stat_strdup_of_os</span> are allocated using <span class="c003">caml_stat_alloc</span>, so they need to
be deallocated using <span class="c003">caml_stat_free</span> when they are no longer needed.</p>
<h5 class="paragraph" id="sec474">Example</h5>
<p> We want to bind the function <span class="c003">getenv</span> in a way that works
both under Unix and Windows. Under Unix this function has the prototype:</p><pre>    char *getenv(const char *);
</pre><p>While the Unicode version under Windows has the prototype:
</p><pre>    WCHAR *_wgetenv(const WCHAR *);
</pre><p>
In terms of <span class="c003">char_os</span>, both functions take an argument of type <span class="c003">char_os *</span> and
return a result of the same type. We begin by choosing the right implementation
of the function to bind:</p><pre>#ifdef _WIN32
#define getenv_os _wgetenv
#else
#define getenv_os getenv
#endif
</pre><p>
The rest of the binding is the same for both platforms:</p><pre>/* The following define is necessary because the API is experimental */
#define CAML_INTERNALS

#include &lt;caml/mlvalues.h&gt;
#include &lt;caml/misc.h&gt;
#include &lt;caml/alloc.h&gt;
#include &lt;caml/fail.h&gt;
#include &lt;caml/osdeps.h&gt;
#include &lt;stdlib.h&gt;

CAMLprim value stub_getenv(value var_name)
{
  CAMLparam1(var_name);
  CAMLlocal1(var_value);
  char_os *var_name_os, *var_value_os;

  var_name_os = caml_stat_strdup_to_os(String_val(var_name));
  var_value_os = getenv_os(var_name_os);
  caml_stat_free(var_name_os);

  if (var_value_os == NULL)
    caml_raise_not_found();

  var_value = caml_copy_string_of_os(var_value_os);

  CAMLreturn(var_value);
}
</pre>
<h2 class="section" id="sec475">20.13&#XA0;&#XA0;Building mixed C/OCaml libraries: <span class="c003">ocamlmklib</span></h2>
<p>
<a id="s-ocamlmklib"></a></p><p>The <span class="c003">ocamlmklib</span> command facilitates the construction of libraries
containing both OCaml code and C code, and usable both in static
linking and dynamic linking modes. This command is available under
Windows since Objective Caml 3.11 and under other operating systems since
Objective Caml 3.03.</p><p>The <span class="c003">ocamlmklib</span> command takes three kinds of arguments:
</p><ul class="itemize"><li class="li-itemize">
OCaml source files and object files (<span class="c003">.cmo</span>, <span class="c003">.cmx</span>, <span class="c003">.ml</span>)
comprising the OCaml part of the library;
</li><li class="li-itemize">C object files (<span class="c003">.o</span>, <span class="c003">.a</span>, respectively, <span class="c003">.obj</span>, <span class="c003">.lib</span>)
comprising the C part of the library;
</li><li class="li-itemize">Support libraries for the C part (<span class="c003">-l</span><span class="c009">lib</span>).
</li></ul><p>
It generates the following outputs:
</p><ul class="itemize"><li class="li-itemize">
An OCaml bytecode library <span class="c003">.cma</span> incorporating the <span class="c003">.cmo</span> and
<span class="c003">.ml</span> OCaml files given as arguments, and automatically referencing the
C library generated with the C object files.
</li><li class="li-itemize">An OCaml native-code library <span class="c003">.cmxa</span> incorporating the <span class="c003">.cmx</span> and
<span class="c003">.ml</span> OCaml files given as arguments, and automatically referencing the
C library generated with the C object files.
</li><li class="li-itemize">If dynamic linking is supported on the target platform, a
<span class="c003">.so</span> (respectively, <span class="c003">.dll</span>) shared library built from the C object files given as arguments,
and automatically referencing the support libraries.
</li><li class="li-itemize">A C static library <span class="c003">.a</span>(respectively, <span class="c003">.lib</span>) built from the C object files.
</li></ul><p>
In addition, the following options are recognized:
</p><dl class="description"><dt class="dt-description">
<span class="c013"><span class="c003">-cclib</span>, <span class="c003">-ccopt</span>, <span class="c003">-I</span>, <span class="c003">-linkall</span></span></dt><dd class="dd-description">
These options are passed as is to <span class="c003">ocamlc</span> or <span class="c003">ocamlopt</span>.
See the documentation of these commands.
</dd><dt class="dt-description"><span class="c013"><span class="c003">-rpath</span>, <span class="c003">-R</span>, <span class="c003">-Wl,-rpath</span>, <span class="c003">-Wl,-R</span></span></dt><dd class="dd-description">
These options are passed as is to the C compiler. Refer to the
documentation of the C compiler.
</dd><dt class="dt-description"><span class="c006">-custom</span></dt><dd class="dd-description"> Force the construction of a statically linked library
only, even if dynamic linking is supported.
</dd><dt class="dt-description"><span class="c006">-failsafe</span></dt><dd class="dd-description"> Fall back to building a statically linked library
if a problem occurs while building the shared library (e.g. some of
the support libraries are not available as shared libraries).
</dd><dt class="dt-description"><span class="c013"><span class="c003">-L</span><span class="c009">dir</span></span></dt><dd class="dd-description"> Add <span class="c009">dir</span> to the search path for support
libraries (<span class="c003">-l</span><span class="c009">lib</span>).
</dd><dt class="dt-description"><span class="c013"><span class="c003">-ocamlc</span> <span class="c009">cmd</span></span></dt><dd class="dd-description"> Use <span class="c009">cmd</span> instead of <span class="c003">ocamlc</span> to call
the bytecode compiler.
</dd><dt class="dt-description"><span class="c013"><span class="c003">-ocamlopt</span> <span class="c009">cmd</span></span></dt><dd class="dd-description"> Use <span class="c009">cmd</span> instead of <span class="c003">ocamlopt</span> to call
the native-code compiler.
</dd><dt class="dt-description"><span class="c013"><span class="c003">-o</span> <span class="c009">output</span></span></dt><dd class="dd-description"> Set the name of the generated OCaml library.
<span class="c003">ocamlmklib</span> will generate <span class="c009">output</span><span class="c003">.cma</span> and/or <span class="c009">output</span><span class="c003">.cmxa</span>.
If not specified, defaults to <span class="c003">a</span>.
</dd><dt class="dt-description"><span class="c013"><span class="c003">-oc</span> <span class="c009">outputc</span></span></dt><dd class="dd-description"> Set the name of the generated C library.
<span class="c003">ocamlmklib</span> will generate <span class="c003">lib</span><span class="c009">outputc</span><span class="c003">.so</span> (if shared
libraries are supported) and <span class="c003">lib</span><span class="c009">outputc</span><span class="c003">.a</span>.
If not specified, defaults to the output name given with <span class="c003">-o</span>.
</dd></dl><p>On native Windows, the following environment variable is also consulted:</p><dl class="description"><dt class="dt-description">
<span class="c006">OCAML_FLEXLINK</span></dt><dd class="dd-description"> Alternative executable to use instead of the
configured value. Primarily used for bootstrapping.
</dd></dl>
<h5 class="paragraph" id="sec476">Example</h5>
<p> Consider an OCaml interface to the standard <span class="c003">libz</span>
C library for reading and writing compressed files. Assume this
library resides in <span class="c003">/usr/local/zlib</span>. This interface is
composed of an OCaml part <span class="c003">zip.cmo</span>/<span class="c003">zip.cmx</span> and a C part <span class="c003">zipstubs.o</span>
containing the stub code around the <span class="c003">libz</span> entry points. The
following command builds the OCaml libraries <span class="c003">zip.cma</span> and <span class="c003">zip.cmxa</span>,
as well as the companion C libraries <span class="c003">dllzip.so</span> and <span class="c003">libzip.a</span>:
</p><pre>ocamlmklib -o zip zip.cmo zip.cmx zipstubs.o -lz -L/usr/local/zlib
</pre><p>If shared libraries are supported, this performs the following
commands:
</p><pre>ocamlc -a -o zip.cma zip.cmo -dllib -lzip \
        -cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib
ocamlopt -a -o zip.cmxa zip.cmx -cclib -lzip \
        -cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib
gcc -shared -o dllzip.so zipstubs.o -lz -L/usr/local/zlib
ar rc libzip.a zipstubs.o
</pre><p>Note: This example is on a Unix system. The exact command lines
may be different on other systems.</p><p>If shared libraries are not supported, the following commands are
performed instead:
</p><pre>ocamlc -a -custom -o zip.cma zip.cmo -cclib -lzip \
        -cclib -lz -ccopt -L/usr/local/zlib
ocamlopt -a -o zip.cmxa zip.cmx -lzip \
        -cclib -lz -ccopt -L/usr/local/zlib
ar rc libzip.a zipstubs.o
</pre><p>Instead of building simultaneously the bytecode library, the
native-code library and the C libraries, <span class="c003">ocamlmklib</span> can be called
three times to build each separately. Thus,
</p><pre>ocamlmklib -o zip zip.cmo -lz -L/usr/local/zlib
</pre><p>builds the bytecode library <span class="c003">zip.cma</span>, and
</p><pre>ocamlmklib -o zip zip.cmx -lz -L/usr/local/zlib
</pre><p>builds the native-code library <span class="c003">zip.cmxa</span>, and
</p><pre>ocamlmklib -o zip zipstubs.o -lz -L/usr/local/zlib
</pre><p>builds the C libraries <span class="c003">dllzip.so</span> and <span class="c003">libzip.a</span>. Notice that the
support libraries (<span class="c003">-lz</span>) and the corresponding options
(<span class="c003">-L/usr/local/zlib</span>) must be given on all three invocations of <span class="c003">ocamlmklib</span>,
because they are needed at different times depending on whether shared
libraries are supported.

</p>
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